Change string compression encoding.

Encode the string compression flag as the least significant
bit of the "count" field, with 0 meaning compressed and 1
meaning uncompressed.

The main vdex file is a tiny bit larger (+28B for prebuilt
boot images, +32 for on-device built images) and the oat
file sizes change. Measured on Nexus 9, AOSP ToT, these
changes are insignificant when string compression is
disabled (-200B for the 32-bit boot*.oat for prebuilt boot
image, -4KiB when built on the device attributable to
rounding, -16B for 64-bit boot*.oat for prebuilt boot image,
no change when built on device) but with string compression
enabled we get significant differences:
  prebuilt multi-part boot image:
    - 32-bit boot*.oat: -28KiB
    - 64-bit boot*.oat: -24KiB
  on-device built single boot image:
    - 32-bit boot.oat: -32KiB
    - 64-bit boot.oat: -28KiB
The boot image oat file overhead for string compression:
  prebuilt multi-part boot image:
    - 32-bit boot*.oat: before: ~80KiB after: ~52KiB
    - 64-bit boot*.oat: before: ~116KiB after: ~92KiB
  on-device built single boot image:
    - 32-bit boot.oat: before: 92KiB after: 60KiB
    - 64-bit boot.oat: before: 116KiB after: 92KiB

The differences in the SplitStringBenchmark seem to be lost
in the noise.

Test: Run ART test suite on host and Nexus 9 with Optimizing.
Test: Run ART test suite on host and Nexus 9 with interpreter.
Test: All of the above with string compression enabled.
Bug: 31040547

Change-Id: I7570c2b700f1a31004a2d3c18b1cc30046d35a74

21 files changed, 550 insertions, 467 deletions

diff --git a/compiler/optimizing/code_generator_arm.cc b/compiler/optimizing/code_generator_arm.cc
index 7cab97d2e5..b2bfe42548 100644
--- a/compiler/optimizing/code_generator_arm.cc
+++ b/compiler/optimizing/code_generator_arm.cc
@@ -4858,16 +4858,21 @@ void InstructionCodeGeneratorARM::VisitArrayGet(HArrayGet* instruction) {
     case Primitive::kPrimShort:
     case Primitive::kPrimChar:
     case Primitive::kPrimInt: {
+      Register length;
+      if (maybe_compressed_char_at) {
+        length = locations->GetTemp(0).AsRegister<Register>();
+        uint32_t count_offset = mirror::String::CountOffset().Uint32Value();
+        __ LoadFromOffset(kLoadWord, length, obj, count_offset);
+        codegen_->MaybeRecordImplicitNullCheck(instruction);
+      }
       if (index.IsConstant()) {
         int32_t const_index = index.GetConstant()->AsIntConstant()->GetValue();
         if (maybe_compressed_char_at) {
-          Register length = IP;
           Label uncompressed_load, done;
-          uint32_t count_offset = mirror::String::CountOffset().Uint32Value();
-          __ LoadFromOffset(kLoadWord, length, obj, count_offset);
-          codegen_->MaybeRecordImplicitNullCheck(instruction);
-          __ cmp(length, ShifterOperand(0));
-          __ b(&uncompressed_load, GE);
+          __ Lsrs(length, length, 1u);  // LSRS has a 16-bit encoding, TST (immediate) does not.
+          static_assert(static_cast<uint32_t>(mirror::StringCompressionFlag::kCompressed) == 0u,
+                        "Expecting 0=compressed, 1=uncompressed");
+          __ b(&uncompressed_load, CS);
           __ LoadFromOffset(kLoadUnsignedByte,
                             out_loc.AsRegister<Register>(),
                             obj,
@@ -4902,12 +4907,10 @@ void InstructionCodeGeneratorARM::VisitArrayGet(HArrayGet* instruction) {
         }
         if (maybe_compressed_char_at) {
           Label uncompressed_load, done;
-          uint32_t count_offset = mirror::String::CountOffset().Uint32Value();
-          Register length = locations->GetTemp(0).AsRegister<Register>();
-          __ LoadFromOffset(kLoadWord, length, obj, count_offset);
-          codegen_->MaybeRecordImplicitNullCheck(instruction);
-          __ cmp(length, ShifterOperand(0));
-          __ b(&uncompressed_load, GE);
+          __ Lsrs(length, length, 1u);  // LSRS has a 16-bit encoding, TST (immediate) does not.
+          static_assert(static_cast<uint32_t>(mirror::StringCompressionFlag::kCompressed) == 0u,
+                        "Expecting 0=compressed, 1=uncompressed");
+          __ b(&uncompressed_load, CS);
           __ ldrb(out_loc.AsRegister<Register>(),
                   Address(temp, index.AsRegister<Register>(), Shift::LSL, 0));
           __ b(&done);
@@ -5312,7 +5315,7 @@ void InstructionCodeGeneratorARM::VisitArrayLength(HArrayLength* instruction) {
   codegen_->MaybeRecordImplicitNullCheck(instruction);
   // Mask out compression flag from String's array length.
   if (mirror::kUseStringCompression && instruction->IsStringLength()) {
-    __ bic(out, out, ShifterOperand(1u << 31));
+    __ Lsr(out, out, 1u);
   }
 }
 
diff --git a/compiler/optimizing/code_generator_arm64.cc b/compiler/optimizing/code_generator_arm64.cc
index d868984387..37322c68f1 100644
--- a/compiler/optimizing/code_generator_arm64.cc
+++ b/compiler/optimizing/code_generator_arm64.cc
@@ -2326,13 +2326,22 @@ void InstructionCodeGeneratorARM64::VisitArrayGet(HArrayGet* instruction) {
     if (maybe_compressed_char_at) {
       uint32_t count_offset = mirror::String::CountOffset().Uint32Value();
       length = temps.AcquireW();
-      __ Ldr(length, HeapOperand(obj, count_offset));
+      if (instruction->GetArray()->IsIntermediateAddress()) {
+        DCHECK_LT(count_offset, offset);
+        int64_t adjusted_offset = static_cast<int64_t>(count_offset) - static_cast<int64_t>(offset);
+        // Note that `adjusted_offset` is negative, so this will be a LDUR.
+        __ Ldr(length, MemOperand(obj.X(), adjusted_offset));
+      } else {
+        __ Ldr(length, HeapOperand(obj, count_offset));
+      }
       codegen_->MaybeRecordImplicitNullCheck(instruction);
     }
     if (index.IsConstant()) {
       if (maybe_compressed_char_at) {
         vixl::aarch64::Label uncompressed_load, done;
-        __ Tbz(length.W(), kWRegSize - 1, &uncompressed_load);
+        static_assert(static_cast<uint32_t>(mirror::StringCompressionFlag::kCompressed) == 0u,
+                      "Expecting 0=compressed, 1=uncompressed");
+        __ Tbnz(length.W(), 0, &uncompressed_load);
         __ Ldrb(Register(OutputCPURegister(instruction)),
                 HeapOperand(obj, offset + Int64ConstantFrom(index)));
         __ B(&done);
@@ -2360,7 +2369,9 @@ void InstructionCodeGeneratorARM64::VisitArrayGet(HArrayGet* instruction) {
       }
       if (maybe_compressed_char_at) {
         vixl::aarch64::Label uncompressed_load, done;
-        __ Tbz(length.W(), kWRegSize - 1, &uncompressed_load);
+        static_assert(static_cast<uint32_t>(mirror::StringCompressionFlag::kCompressed) == 0u,
+                      "Expecting 0=compressed, 1=uncompressed");
+        __ Tbnz(length.W(), 0, &uncompressed_load);
         __ Ldrb(Register(OutputCPURegister(instruction)),
                 HeapOperand(temp, XRegisterFrom(index), LSL, 0));
         __ B(&done);
@@ -2405,7 +2416,7 @@ void InstructionCodeGeneratorARM64::VisitArrayLength(HArrayLength* instruction)
   codegen_->MaybeRecordImplicitNullCheck(instruction);
   // Mask out compression flag from String's array length.
   if (mirror::kUseStringCompression && instruction->IsStringLength()) {
-    __ And(out.W(), out.W(), Operand(static_cast<int32_t>(INT32_MAX)));
+    __ Lsr(out.W(), out.W(), 1u);
   }
 }
 
diff --git a/compiler/optimizing/code_generator_arm_vixl.cc b/compiler/optimizing/code_generator_arm_vixl.cc
index e69528e43f..254d90898a 100644
--- a/compiler/optimizing/code_generator_arm_vixl.cc
+++ b/compiler/optimizing/code_generator_arm_vixl.cc
@@ -3790,16 +3790,21 @@ void InstructionCodeGeneratorARMVIXL::VisitArrayGet(HArrayGet* instruction) {
     case Primitive::kPrimShort:
     case Primitive::kPrimChar:
     case Primitive::kPrimInt: {
+      vixl32::Register length;
+      if (maybe_compressed_char_at) {
+        length = RegisterFrom(locations->GetTemp(0));
+        uint32_t count_offset = mirror::String::CountOffset().Uint32Value();
+        GetAssembler()->LoadFromOffset(kLoadWord, length, obj, count_offset);
+        codegen_->MaybeRecordImplicitNullCheck(instruction);
+      }
       if (index.IsConstant()) {
         int32_t const_index = index.GetConstant()->AsIntConstant()->GetValue();
         if (maybe_compressed_char_at) {
-          vixl32::Register length = temps.Acquire();
           vixl32::Label uncompressed_load, done;
-          uint32_t count_offset = mirror::String::CountOffset().Uint32Value();
-          GetAssembler()->LoadFromOffset(kLoadWord, length, obj, count_offset);
-          codegen_->MaybeRecordImplicitNullCheck(instruction);
-          __ Cmp(length, 0);
-          __ B(ge, &uncompressed_load);
+          __ Lsrs(length, length, 1u);  // LSRS has a 16-bit encoding, TST (immediate) does not.
+          static_assert(static_cast<uint32_t>(mirror::StringCompressionFlag::kCompressed) == 0u,
+                        "Expecting 0=compressed, 1=uncompressed");
+          __ B(cs, &uncompressed_load);
           GetAssembler()->LoadFromOffset(kLoadUnsignedByte,
                                          RegisterFrom(out_loc),
                                          obj,
@@ -3827,12 +3832,10 @@ void InstructionCodeGeneratorARMVIXL::VisitArrayGet(HArrayGet* instruction) {
         }
         if (maybe_compressed_char_at) {
           vixl32::Label uncompressed_load, done;
-          uint32_t count_offset = mirror::String::CountOffset().Uint32Value();
-          vixl32::Register length = RegisterFrom(locations->GetTemp(0));
-          GetAssembler()->LoadFromOffset(kLoadWord, length, obj, count_offset);
-          codegen_->MaybeRecordImplicitNullCheck(instruction);
-          __ Cmp(length, 0);
-          __ B(ge, &uncompressed_load);
+          __ Lsrs(length, length, 1u);  // LSRS has a 16-bit encoding, TST (immediate) does not.
+          static_assert(static_cast<uint32_t>(mirror::StringCompressionFlag::kCompressed) == 0u,
+                        "Expecting 0=compressed, 1=uncompressed");
+          __ B(cs, &uncompressed_load);
           __ Ldrb(RegisterFrom(out_loc), MemOperand(temp, RegisterFrom(index), vixl32::LSL, 0));
           __ B(&done);
           __ Bind(&uncompressed_load);
@@ -4211,7 +4214,7 @@ void InstructionCodeGeneratorARMVIXL::VisitArrayLength(HArrayLength* instruction
   codegen_->MaybeRecordImplicitNullCheck(instruction);
   // Mask out compression flag from String's array length.
   if (mirror::kUseStringCompression && instruction->IsStringLength()) {
-    __ Bic(out, out, 1u << 31);
+    __ Lsr(out, out, 1u);
   }
 }
 
diff --git a/compiler/optimizing/code_generator_x86.cc b/compiler/optimizing/code_generator_x86.cc
index 2f946e4263..2782d567ad 100644
--- a/compiler/optimizing/code_generator_x86.cc
+++ b/compiler/optimizing/code_generator_x86.cc
@@ -151,7 +151,7 @@ class BoundsCheckSlowPathX86 : public SlowPathCode {
       }
       __ movl(length_loc.AsRegister<Register>(), array_len);
       if (mirror::kUseStringCompression) {
-        __ andl(length_loc.AsRegister<Register>(), Immediate(INT32_MAX));
+        __ shrl(length_loc.AsRegister<Register>(), Immediate(1));
       }
     }
     x86_codegen->EmitParallelMoves(
@@ -5237,9 +5237,11 @@ void InstructionCodeGeneratorX86::VisitArrayGet(HArrayGet* instruction) {
         // Branch cases into compressed and uncompressed for each index's type.
         uint32_t count_offset = mirror::String::CountOffset().Uint32Value();
         NearLabel done, not_compressed;
-        __ cmpl(Address(obj, count_offset), Immediate(0));
+        __ testl(Address(obj, count_offset), Immediate(1));
         codegen_->MaybeRecordImplicitNullCheck(instruction);
-        __ j(kGreaterEqual, &not_compressed);
+        static_assert(static_cast<uint32_t>(mirror::StringCompressionFlag::kCompressed) == 0u,
+                      "Expecting 0=compressed, 1=uncompressed");
+        __ j(kNotZero, &not_compressed);
         __ movzxb(out, CodeGeneratorX86::ArrayAddress(obj, index, TIMES_1, data_offset));
         __ jmp(&done);
         __ Bind(&not_compressed);
@@ -5589,7 +5591,7 @@ void InstructionCodeGeneratorX86::VisitArrayLength(HArrayLength* instruction) {
   codegen_->MaybeRecordImplicitNullCheck(instruction);
   // Mask out most significant bit in case the array is String's array of char.
   if (mirror::kUseStringCompression && instruction->IsStringLength()) {
-    __ andl(out, Immediate(INT32_MAX));
+    __ shrl(out, Immediate(1));
   }
 }
 
@@ -5648,10 +5650,12 @@ void InstructionCodeGeneratorX86::VisitBoundsCheck(HBoundsCheck* instruction) {
       Location array_loc = array_length->GetLocations()->InAt(0);
       Address array_len(array_loc.AsRegister<Register>(), len_offset);
       if (is_string_compressed_char_at) {
+        // TODO: if index_loc.IsConstant(), compare twice the index (to compensate for
+        // the string compression flag) with the in-memory length and avoid the temporary.
         Register length_reg = locations->GetTemp(0).AsRegister<Register>();
         __ movl(length_reg, array_len);
         codegen_->MaybeRecordImplicitNullCheck(array_length);
-        __ andl(length_reg, Immediate(INT32_MAX));
+        __ shrl(length_reg, Immediate(1));
         codegen_->GenerateIntCompare(length_reg, index_loc);
       } else {
         // Checking bounds for general case:
diff --git a/compiler/optimizing/code_generator_x86_64.cc b/compiler/optimizing/code_generator_x86_64.cc
index 232c3b3cbb..a3327e261c 100644
--- a/compiler/optimizing/code_generator_x86_64.cc
+++ b/compiler/optimizing/code_generator_x86_64.cc
@@ -199,7 +199,7 @@ class BoundsCheckSlowPathX86_64 : public SlowPathCode {
       }
       __ movl(length_loc.AsRegister<CpuRegister>(), array_len);
       if (mirror::kUseStringCompression) {
-        __ andl(length_loc.AsRegister<CpuRegister>(), Immediate(INT32_MAX));
+        __ shrl(length_loc.AsRegister<CpuRegister>(), Immediate(1));
       }
     }
 
@@ -4729,9 +4729,11 @@ void InstructionCodeGeneratorX86_64::VisitArrayGet(HArrayGet* instruction) {
         // Branch cases into compressed and uncompressed for each index's type.
         uint32_t count_offset = mirror::String::CountOffset().Uint32Value();
         NearLabel done, not_compressed;
-        __ cmpl(Address(obj, count_offset), Immediate(0));
+        __ testl(Address(obj, count_offset), Immediate(1));
         codegen_->MaybeRecordImplicitNullCheck(instruction);
-        __ j(kGreaterEqual, &not_compressed);
+        static_assert(static_cast<uint32_t>(mirror::StringCompressionFlag::kCompressed) == 0u,
+                      "Expecting 0=compressed, 1=uncompressed");
+        __ j(kNotZero, &not_compressed);
         __ movzxb(out, CodeGeneratorX86_64::ArrayAddress(obj, index, TIMES_1, data_offset));
         __ jmp(&done);
         __ Bind(&not_compressed);
@@ -5063,7 +5065,7 @@ void InstructionCodeGeneratorX86_64::VisitArrayLength(HArrayLength* instruction)
   codegen_->MaybeRecordImplicitNullCheck(instruction);
   // Mask out most significant bit in case the array is String's array of char.
   if (mirror::kUseStringCompression && instruction->IsStringLength()) {
-    __ andl(out, Immediate(INT32_MAX));
+    __ shrl(out, Immediate(1));
   }
 }
 
@@ -5115,10 +5117,12 @@ void InstructionCodeGeneratorX86_64::VisitBoundsCheck(HBoundsCheck* instruction)
       Location array_loc = array_length->GetLocations()->InAt(0);
       Address array_len(array_loc.AsRegister<CpuRegister>(), len_offset);
       if (mirror::kUseStringCompression && instruction->IsStringCharAt()) {
+        // TODO: if index_loc.IsConstant(), compare twice the index (to compensate for
+        // the string compression flag) with the in-memory length and avoid the temporary.
         CpuRegister length_reg = CpuRegister(TMP);
         __ movl(length_reg, array_len);
         codegen_->MaybeRecordImplicitNullCheck(array_length);
-        __ andl(length_reg, Immediate(INT32_MAX));
+        __ shrl(length_reg, Immediate(1));
         codegen_->GenerateIntCompare(length_reg, index_loc);
       } else {
         // Checking the bound for general case:
diff --git a/compiler/optimizing/instruction_simplifier_arm64.cc b/compiler/optimizing/instruction_simplifier_arm64.cc
index d0dd650024..6d107d571f 100644
--- a/compiler/optimizing/instruction_simplifier_arm64.cc
+++ b/compiler/optimizing/instruction_simplifier_arm64.cc
@@ -140,13 +140,6 @@ void InstructionSimplifierArm64Visitor::VisitAnd(HAnd* instruction) {
 
 void InstructionSimplifierArm64Visitor::VisitArrayGet(HArrayGet* instruction) {
   size_t data_offset = CodeGenerator::GetArrayDataOffset(instruction);
-  // Don't move the array pointer if it is charAt because we need to take the count first.
-  // TODO: Implement reading (length + compression) for String compression feature from
-  // negative offset (count_offset - data_offset) using LDP and clobbering an extra temporary.
-  // Note that "LDR (Immediate)" does not have a "signed offset" encoding.
-  if (mirror::kUseStringCompression && instruction->IsStringCharAt()) {
-    return;
-  }
   if (TryExtractArrayAccessAddress(instruction,
                                    instruction->GetArray(),
                                    instruction->GetIndex(),
diff --git a/compiler/optimizing/intrinsics_arm.cc b/compiler/optimizing/intrinsics_arm.cc
index 93a2340a32..3e6b0afb3f 100644
--- a/compiler/optimizing/intrinsics_arm.cc
+++ b/compiler/optimizing/intrinsics_arm.cc
@@ -1058,7 +1058,6 @@ void IntrinsicLocationsBuilderARM::VisitStringCompareTo(HInvoke* invoke) {
   // Need temporary registers for String compression's feature.
   if (mirror::kUseStringCompression) {
     locations->AddTemp(Location::RequiresRegister());
-    locations->AddTemp(Location::RequiresRegister());
   }
   locations->SetOut(Location::RequiresRegister(), Location::kOutputOverlap);
 }
@@ -1074,10 +1073,9 @@ void IntrinsicCodeGeneratorARM::VisitStringCompareTo(HInvoke* invoke) {
   Register temp0 = locations->GetTemp(0).AsRegister<Register>();
   Register temp1 = locations->GetTemp(1).AsRegister<Register>();
   Register temp2 = locations->GetTemp(2).AsRegister<Register>();
-  Register temp3, temp4;
+  Register temp3;
   if (mirror::kUseStringCompression) {
     temp3 = locations->GetTemp(3).AsRegister<Register>();
-    temp4 = locations->GetTemp(4).AsRegister<Register>();
   }
 
   Label loop;
@@ -1104,41 +1102,42 @@ void IntrinsicCodeGeneratorARM::VisitStringCompareTo(HInvoke* invoke) {
   // Reference equality check, return 0 if same reference.
   __ subs(out, str, ShifterOperand(arg));
   __ b(&end, EQ);
+
   if (mirror::kUseStringCompression) {
-    // Load lengths of this and argument strings.
+    // Load `count` fields of this and argument strings.
     __ ldr(temp3, Address(str, count_offset));
-    __ ldr(temp4, Address(arg, count_offset));
-    // Clean out compression flag from lengths.
-    __ bic(temp0, temp3, ShifterOperand(0x80000000));
-    __ bic(IP, temp4, ShifterOperand(0x80000000));
+    __ ldr(temp2, Address(arg, count_offset));
+    // Extract lengths from the `count` fields.
+    __ Lsr(temp0, temp3, 1u);
+    __ Lsr(temp1, temp2, 1u);
   } else {
     // Load lengths of this and argument strings.
     __ ldr(temp0, Address(str, count_offset));
-    __ ldr(IP, Address(arg, count_offset));
+    __ ldr(temp1, Address(arg, count_offset));
   }
   // out = length diff.
-  __ subs(out, temp0, ShifterOperand(IP));
+  __ subs(out, temp0, ShifterOperand(temp1));
   // temp0 = min(len(str), len(arg)).
   __ it(GT);
-  __ mov(temp0, ShifterOperand(IP), GT);
+  __ mov(temp0, ShifterOperand(temp1), GT);
   // Shorter string is empty?
   __ CompareAndBranchIfZero(temp0, &end);
 
   if (mirror::kUseStringCompression) {
     // Check if both strings using same compression style to use this comparison loop.
-    __ eors(temp3, temp3, ShifterOperand(temp4));
-    __ b(&different_compression, MI);
-  }
-  // Store offset of string value in preparation for comparison loop.
-  __ mov(temp1, ShifterOperand(value_offset));
-  if (mirror::kUseStringCompression) {
+    __ eor(temp2, temp2, ShifterOperand(temp3));
+    __ Lsrs(temp2, temp2, 1u);
+    __ b(&different_compression, CS);
     // For string compression, calculate the number of bytes to compare (not chars).
     // This could in theory exceed INT32_MAX, so treat temp0 as unsigned.
-    __ cmp(temp4, ShifterOperand(0));
-    __ it(GE);
-    __ add(temp0, temp0, ShifterOperand(temp0), GE);
+    __ Lsls(temp3, temp3, 31u);  // Extract purely the compression flag.
+    __ it(NE);
+    __ add(temp0, temp0, ShifterOperand(temp0), NE);
   }
 
+  // Store offset of string value in preparation for comparison loop.
+  __ mov(temp1, ShifterOperand(value_offset));
+
   // Assertions that must hold in order to compare multiple characters at a time.
   CHECK_ALIGNED(value_offset, 8);
   static_assert(IsAligned<8>(kObjectAlignment),
@@ -1198,69 +1197,80 @@ void IntrinsicCodeGeneratorARM::VisitStringCompareTo(HInvoke* invoke) {
   // The comparison is unsigned for string compression, otherwise signed.
   __ cmp(temp0, ShifterOperand(temp1, LSR, mirror::kUseStringCompression ? 3 : 4));
   __ b(&end, mirror::kUseStringCompression ? LS : LE);
+
   // Extract the characters and calculate the difference.
-  Label uncompressed_string, continue_process;
   if (mirror::kUseStringCompression) {
-    __ cmp(temp4, ShifterOperand(0));
-    __ b(&uncompressed_string, GE);
-    __ bic(temp1, temp1, ShifterOperand(0x7));
-    __ b(&continue_process);
+    // For compressed strings we need to clear 0x7 from temp1, for uncompressed we need to clear
+    // 0xf. We also need to prepare the character extraction mask `uncompressed ? 0xffffu : 0xffu`.
+    // The compression flag is now in the highest bit of temp3, so let's play some tricks.
+    __ orr(temp3, temp3, ShifterOperand(0xffu << 23));  // uncompressed ? 0xff800000u : 0x7ff80000u
+    __ bic(temp1, temp1, ShifterOperand(temp3, LSR, 31 - 3));  // &= ~(uncompressed ? 0xfu : 0x7u)
+    __ Asr(temp3, temp3, 7u);                           // uncompressed ? 0xffff0000u : 0xff0000u.
+    __ Lsr(temp2, temp2, temp1);                        // Extract second character.
+    __ Lsr(temp3, temp3, 16u);                          // uncompressed ? 0xffffu : 0xffu
+    __ Lsr(out, IP, temp1);                             // Extract first character.
+    __ and_(temp2, temp2, ShifterOperand(temp3));
+    __ and_(out, out, ShifterOperand(temp3));
+  } else {
+    __ bic(temp1, temp1, ShifterOperand(0xf));
+    __ Lsr(temp2, temp2, temp1);
+    __ Lsr(out, IP, temp1);
+    __ movt(temp2, 0);
+    __ movt(out, 0);
   }
-  __ Bind(&uncompressed_string);
-  __ bic(temp1, temp1, ShifterOperand(0xf));
-  __ Bind(&continue_process);
 
-  __ Lsr(temp2, temp2, temp1);
-  __ Lsr(IP, IP, temp1);
-  Label calculate_difference, uncompressed_string_extract_chars;
-  if (mirror::kUseStringCompression) {
-    __ cmp(temp4, ShifterOperand(0));
-    __ b(&uncompressed_string_extract_chars, GE);
-    __ ubfx(temp2, temp2, 0, 8);
-    __ ubfx(IP, IP, 0, 8);
-    __ b(&calculate_difference);
-  }
-  __ Bind(&uncompressed_string_extract_chars);
-  __ movt(temp2, 0);
-  __ movt(IP, 0);
-  __ Bind(&calculate_difference);
-  __ sub(out, IP, ShifterOperand(temp2));
-  __ b(&end);
+  __ sub(out, out, ShifterOperand(temp2));
 
   if (mirror::kUseStringCompression) {
+    __ b(&end);
+    __ Bind(&different_compression);
+
+    // Comparison for different compression style.
     const size_t c_char_size = Primitive::ComponentSize(Primitive::kPrimByte);
     DCHECK_EQ(c_char_size, 1u);
-    Label loop_arg_compressed, loop_this_compressed, find_diff;
-    // Comparison for different compression style.
-    // This part is when THIS is compressed and ARG is not.
-    __ Bind(&different_compression);
-    __ add(temp2, str, ShifterOperand(value_offset));
-    __ add(temp3, arg, ShifterOperand(value_offset));
-    __ cmp(temp4, ShifterOperand(0));
-    __ b(&loop_arg_compressed, LT);
-
-    __ Bind(&loop_this_compressed);
-    __ ldrb(IP, Address(temp2, c_char_size, Address::PostIndex));
-    __ ldrh(temp4, Address(temp3, char_size, Address::PostIndex));
-    __ cmp(IP, ShifterOperand(temp4));
-    __ b(&find_diff, NE);
-    __ subs(temp0, temp0, ShifterOperand(1));
-    __ b(&loop_this_compressed, GT);
-    __ b(&end);
 
-    // This part is when THIS is not compressed and ARG is.
-    __ Bind(&loop_arg_compressed);
-    __ ldrh(IP, Address(temp2, char_size, Address::PostIndex));
-    __ ldrb(temp4, Address(temp3, c_char_size, Address::PostIndex));
-    __ cmp(IP, ShifterOperand(temp4));
-    __ b(&find_diff, NE);
-    __ subs(temp0, temp0, ShifterOperand(1));
-    __ b(&loop_arg_compressed, GT);
+    // We want to free up the temp3, currently holding `str.count`, for comparison.
+    // So, we move it to the bottom bit of the iteration count `temp0` which we tnen
+    // need to treat as unsigned. Start by freeing the bit with an ADD and continue
+    // further down by a LSRS+SBC which will flip the meaning of the flag but allow
+    // `subs temp0, #2; bhi different_compression_loop` to serve as the loop condition.
+    __ add(temp0, temp0, ShifterOperand(temp0));  // Unlike LSL, this ADD is always 16-bit.
+    // `temp1` will hold the compressed data pointer, `temp2` the uncompressed data pointer.
+    __ mov(temp1, ShifterOperand(str));
+    __ mov(temp2, ShifterOperand(arg));
+    __ Lsrs(temp3, temp3, 1u);                // Continue the move of the compression flag.
+    __ it(CS, kItThen);                       // Interleave with selection of temp1 and temp2.
+    __ mov(temp1, ShifterOperand(arg), CS);   // Preserves flags.
+    __ mov(temp2, ShifterOperand(str), CS);   // Preserves flags.
+    __ sbc(temp0, temp0, ShifterOperand(0));  // Complete the move of the compression flag.
+
+    // Adjust temp1 and temp2 from string pointers to data pointers.
+    __ add(temp1, temp1, ShifterOperand(value_offset));
+    __ add(temp2, temp2, ShifterOperand(value_offset));
+
+    Label different_compression_loop;
+    Label different_compression_diff;
+
+    // Main loop for different compression.
+    __ Bind(&different_compression_loop);
+    __ ldrb(IP, Address(temp1, c_char_size, Address::PostIndex));
+    __ ldrh(temp3, Address(temp2, char_size, Address::PostIndex));
+    __ cmp(IP, ShifterOperand(temp3));
+    __ b(&different_compression_diff, NE);
+    __ subs(temp0, temp0, ShifterOperand(2));
+    __ b(&different_compression_loop, HI);
     __ b(&end);
 
     // Calculate the difference.
-    __ Bind(&find_diff);
-    __ sub(out, IP, ShifterOperand(temp4));
+    __ Bind(&different_compression_diff);
+    __ sub(out, IP, ShifterOperand(temp3));
+    // Flip the difference if the `arg` is compressed.
+    // `temp0` contains inverted `str` compression flag, i.e the same as `arg` compression flag.
+    __ Lsrs(temp0, temp0, 1u);
+    static_assert(static_cast<uint32_t>(mirror::StringCompressionFlag::kCompressed) == 0u,
+                  "Expecting 0=compressed, 1=uncompressed");
+    __ it(CC);
+    __ rsb(out, out, ShifterOperand(0), CC);
   }
 
   __ Bind(&end);
@@ -1298,7 +1308,7 @@ void IntrinsicCodeGeneratorARM::VisitStringEquals(HInvoke* invoke) {
   Register temp1 = locations->GetTemp(1).AsRegister<Register>();
   Register temp2 = locations->GetTemp(2).AsRegister<Register>();
 
-  Label loop, preloop;
+  Label loop;
   Label end;
   Label return_true;
   Label return_false;
@@ -1317,6 +1327,10 @@ void IntrinsicCodeGeneratorARM::VisitStringEquals(HInvoke* invoke) {
     __ CompareAndBranchIfZero(arg, &return_false);
   }
 
+  // Reference equality check, return true if same reference.
+  __ cmp(str, ShifterOperand(arg));
+  __ b(&return_true, EQ);
+
   if (!optimizations.GetArgumentIsString()) {
     // Instanceof check for the argument by comparing class fields.
     // All string objects must have the same type since String cannot be subclassed.
@@ -1328,48 +1342,44 @@ void IntrinsicCodeGeneratorARM::VisitStringEquals(HInvoke* invoke) {
     __ b(&return_false, NE);
   }
 
-  // Load lengths of this and argument strings.
+  // Load `count` fields of this and argument strings.
   __ ldr(temp, Address(str, count_offset));
   __ ldr(temp1, Address(arg, count_offset));
-  // Check if lengths are equal, return false if they're not.
+  // Check if `count` fields are equal, return false if they're not.
   // Also compares the compression style, if differs return false.
   __ cmp(temp, ShifterOperand(temp1));
   __ b(&return_false, NE);
-  // Return true if both strings are empty.
-  if (mirror::kUseStringCompression) {
-    // Length needs to be masked out first because 0 is treated as compressed.
-    __ bic(temp, temp, ShifterOperand(0x80000000));
-  }
+  // Return true if both strings are empty. Even with string compression `count == 0` means empty.
+  static_assert(static_cast<uint32_t>(mirror::StringCompressionFlag::kCompressed) == 0u,
+                "Expecting 0=compressed, 1=uncompressed");
   __ cbz(temp, &return_true);
-  // Reference equality check, return true if same reference.
-  __ cmp(str, ShifterOperand(arg));
-  __ b(&return_true, EQ);
 
-  // Assertions that must hold in order to compare strings 2 characters at a time.
+  // Assertions that must hold in order to compare strings 4 bytes at a time.
   DCHECK_ALIGNED(value_offset, 4);
   static_assert(IsAligned<4>(kObjectAlignment), "String data must be aligned for fast compare.");
 
   if (mirror::kUseStringCompression) {
-    // If not compressed, directly to fast compare. Else do preprocess on length.
-    __ cmp(temp1, ShifterOperand(0));
-    __ b(&preloop, GT);
-    // Mask out compression flag and adjust length for compressed string (8-bit)
-    // as if it is a 16-bit data, new_length = (length + 1) / 2.
-    __ add(temp, temp, ShifterOperand(1));
-    __ Lsr(temp, temp, 1);
-    __ Bind(&preloop);
+    // For string compression, calculate the number of bytes to compare (not chars).
+    // This could in theory exceed INT32_MAX, so treat temp as unsigned.
+    __ Lsrs(temp, temp, 1u);                        // Extract length and check compression flag.
+    __ it(CS);                                      // If uncompressed,
+    __ add(temp, temp, ShifterOperand(temp), CS);   //   double the byte count.
   }
-  // Loop to compare strings 2 characters at a time starting at the front of the string.
-  // Ok to do this because strings with an odd length are zero-padded.
+
+  // Store offset of string value in preparation for comparison loop.
   __ LoadImmediate(temp1, value_offset);
+
+  // Loop to compare strings 4 bytes at a time starting at the front of the string.
+  // Ok to do this because strings are zero-padded to kObjectAlignment.
   __ Bind(&loop);
   __ ldr(out, Address(str, temp1));
   __ ldr(temp2, Address(arg, temp1));
+  __ add(temp1, temp1, ShifterOperand(sizeof(uint32_t)));
   __ cmp(out, ShifterOperand(temp2));
   __ b(&return_false, NE);
-  __ add(temp1, temp1, ShifterOperand(sizeof(uint32_t)));
-  __ subs(temp, temp, ShifterOperand(sizeof(uint32_t) /  sizeof(uint16_t)));
-  __ b(&loop, GT);
+  // With string compression, we have compared 4 bytes, otherwise 2 chars.
+  __ subs(temp, temp, ShifterOperand(mirror::kUseStringCompression ? 4 : 2));
+  __ b(&loop, HI);
 
   // Return true and exit the function.
   // If loop does not result in returning false, we return true.
@@ -2477,8 +2487,8 @@ void IntrinsicCodeGeneratorARM::VisitStringGetCharsNoCheck(HInvoke* invoke) {
     const uint32_t count_offset = mirror::String::CountOffset().Uint32Value();
     // String's length.
     __ ldr(IP, Address(srcObj, count_offset));
-    __ cmp(IP, ShifterOperand(0));
-    __ b(&compressed_string_preloop, LT);
+    __ tst(IP, ShifterOperand(1));
+    __ b(&compressed_string_preloop, EQ);
   }
   __ add(src_ptr, src_ptr, ShifterOperand(srcBegin, LSL, 1));
 
@@ -2513,9 +2523,10 @@ void IntrinsicCodeGeneratorARM::VisitStringGetCharsNoCheck(HInvoke* invoke) {
   __ subs(num_chr, num_chr, ShifterOperand(1));
   __ strh(IP, Address(dst_ptr, char_size, Address::PostIndex));
   __ b(&remainder, GT);
-  __ b(&done);
 
   if (mirror::kUseStringCompression) {
+    __ b(&done);
+
     const size_t c_char_size = Primitive::ComponentSize(Primitive::kPrimByte);
     DCHECK_EQ(c_char_size, 1u);
     // Copy loop for compressed src, copying 1 character (8-bit) to (16-bit) at a time.
diff --git a/compiler/optimizing/intrinsics_arm64.cc b/compiler/optimizing/intrinsics_arm64.cc
index 47e6d9699d..9b17931d74 100644
--- a/compiler/optimizing/intrinsics_arm64.cc
+++ b/compiler/optimizing/intrinsics_arm64.cc
@@ -1243,7 +1243,6 @@ void IntrinsicLocationsBuilderARM64::VisitStringCompareTo(HInvoke* invoke) {
   // Need temporary registers for String compression's feature.
   if (mirror::kUseStringCompression) {
     locations->AddTemp(Location::RequiresRegister());
-    locations->AddTemp(Location::RequiresRegister());
   }
   locations->SetOut(Location::RequiresRegister(), Location::kOutputOverlap);
 }
@@ -1261,10 +1260,9 @@ void IntrinsicCodeGeneratorARM64::VisitStringCompareTo(HInvoke* invoke) {
   Register temp0 = WRegisterFrom(locations->GetTemp(0));
   Register temp1 = WRegisterFrom(locations->GetTemp(1));
   Register temp2 = WRegisterFrom(locations->GetTemp(2));
-  Register temp3, temp5;
+  Register temp3;
   if (mirror::kUseStringCompression) {
     temp3 = WRegisterFrom(locations->GetTemp(3));
-    temp5 = WRegisterFrom(locations->GetTemp(4));
   }
 
   vixl::aarch64::Label loop;
@@ -1291,68 +1289,65 @@ void IntrinsicCodeGeneratorARM64::VisitStringCompareTo(HInvoke* invoke) {
   // Reference equality check, return 0 if same reference.
   __ Subs(out, str, arg);
   __ B(&end, eq);
+
   if (mirror::kUseStringCompression) {
-    // Load lengths of this and argument strings.
+    // Load `count` fields of this and argument strings.
     __ Ldr(temp3, HeapOperand(str, count_offset));
-    __ Ldr(temp5, HeapOperand(arg, count_offset));
+    __ Ldr(temp2, HeapOperand(arg, count_offset));
     // Clean out compression flag from lengths.
-    __ Bic(temp0, temp3, Operand(static_cast<int32_t>(0x80000000)));
-    __ Bic(temp1, temp5, Operand(static_cast<int32_t>(0x80000000)));
+    __ Lsr(temp0, temp3, 1u);
+    __ Lsr(temp1, temp2, 1u);
   } else {
     // Load lengths of this and argument strings.
     __ Ldr(temp0, HeapOperand(str, count_offset));
     __ Ldr(temp1, HeapOperand(arg, count_offset));
   }
-  // Return zero if both strings are empty.
-  __ Orr(out, temp0, temp1);
-  __ Cbz(out, &end);
   // out = length diff.
   __ Subs(out, temp0, temp1);
-  // temp2 = min(len(str), len(arg)).
-  __ Csel(temp2, temp1, temp0, ge);
+  // temp0 = min(len(str), len(arg)).
+  __ Csel(temp0, temp1, temp0, ge);
   // Shorter string is empty?
-  __ Cbz(temp2, &end);
+  __ Cbz(temp0, &end);
 
   if (mirror::kUseStringCompression) {
     // Check if both strings using same compression style to use this comparison loop.
-    __ Eor(temp3.W(), temp3, Operand(temp5));
-    __ Tbnz(temp3.W(), kWRegSize - 1, &different_compression);
+    __ Eor(temp2, temp2, Operand(temp3));
+    // Interleave with compression flag extraction which is needed for both paths
+    // and also set flags which is needed only for the different compressions path.
+    __ Ands(temp3.W(), temp3.W(), Operand(1));
+    __ Tbnz(temp2, 0, &different_compression);  // Does not use flags.
   }
   // Store offset of string value in preparation for comparison loop.
   __ Mov(temp1, value_offset);
   if (mirror::kUseStringCompression) {
     // For string compression, calculate the number of bytes to compare (not chars).
-    // This could be in theory exceed INT32_MAX, so treat temp2 as unsigned.
-    vixl::aarch64::Label let_it_signed;
-    __ Cmp(temp5, Operand(0));
-    __ B(lt, &let_it_signed);
-    __ Add(temp2, temp2, Operand(temp2));
-    __ Bind(&let_it_signed);
+    // This could in theory exceed INT32_MAX, so treat temp0 as unsigned.
+    __ Lsl(temp0, temp0, temp3);
   }
 
   UseScratchRegisterScope scratch_scope(masm);
   Register temp4 = scratch_scope.AcquireX();
 
-  // Assertions that must hold in order to compare strings 4 characters at a time.
+  // Assertions that must hold in order to compare strings 8 bytes at a time.
   DCHECK_ALIGNED(value_offset, 8);
   static_assert(IsAligned<8>(kObjectAlignment), "String of odd length is not zero padded");
 
   const size_t char_size = Primitive::ComponentSize(Primitive::kPrimChar);
   DCHECK_EQ(char_size, 2u);
 
-  // Promote temp0 to an X reg, ready for LDR.
-  temp0 = temp0.X();
+  // Promote temp2 to an X reg, ready for LDR.
+  temp2 = temp2.X();
 
   // Loop to compare 4x16-bit characters at a time (ok because of string data alignment).
   __ Bind(&loop);
   __ Ldr(temp4, MemOperand(str.X(), temp1.X()));
-  __ Ldr(temp0, MemOperand(arg.X(), temp1.X()));
-  __ Cmp(temp4, temp0);
+  __ Ldr(temp2, MemOperand(arg.X(), temp1.X()));
+  __ Cmp(temp4, temp2);
   __ B(ne, &find_char_diff);
   __ Add(temp1, temp1, char_size * 4);
   // With string compression, we have compared 8 bytes, otherwise 4 chars.
-  __ Subs(temp2, temp2, (mirror::kUseStringCompression) ? 8 : 4);
-  __ B(hi, &loop);
+  __ Subs(temp0, temp0, (mirror::kUseStringCompression) ? 8 : 4);
+  __ B(&loop, hi);
   __ B(&end);
 
   // Promote temp1 to an X reg, ready for EOR.
@@ -1361,78 +1356,85 @@ void IntrinsicCodeGeneratorARM64::VisitStringCompareTo(HInvoke* invoke) {
   // Find the single character difference.
   __ Bind(&find_char_diff);
   // Get the bit position of the first character that differs.
-  __ Eor(temp1, temp0, temp4);
+  __ Eor(temp1, temp2, temp4);
   __ Rbit(temp1, temp1);
   __ Clz(temp1, temp1);
+
   // If the number of chars remaining <= the index where the difference occurs (0-3), then
   // the difference occurs outside the remaining string data, so just return length diff (out).
   // Unlike ARM, we're doing the comparison in one go here, without the subtraction at the
   // find_char_diff_2nd_cmp path, so it doesn't matter whether the comparison is signed or
   // unsigned when string compression is disabled.
   // When it's enabled, the comparison must be unsigned.
-  __ Cmp(temp2, Operand(temp1.W(), LSR, (mirror::kUseStringCompression) ? 3 : 4));
+  __ Cmp(temp0, Operand(temp1.W(), LSR, (mirror::kUseStringCompression) ? 3 : 4));
   __ B(ls, &end);
+
   // Extract the characters and calculate the difference.
-  vixl::aarch64::Label uncompressed_string, continue_process;
   if (mirror:: kUseStringCompression) {
-    __ Tbz(temp5, kWRegSize - 1, &uncompressed_string);
     __ Bic(temp1, temp1, 0x7);
-    __ B(&continue_process);
+    __ Bic(temp1, temp1, Operand(temp3.X(), LSL, 3u));
+  } else {
+    __ Bic(temp1, temp1, 0xf);
   }
-  __ Bind(&uncompressed_string);
-  __ Bic(temp1, temp1, 0xf);
-  __ Bind(&continue_process);
-
-  __ Lsr(temp0, temp0, temp1);
+  __ Lsr(temp2, temp2, temp1);
   __ Lsr(temp4, temp4, temp1);
-  vixl::aarch64::Label uncompressed_string_extract_chars;
   if (mirror::kUseStringCompression) {
-    __ Tbz(temp5, kWRegSize - 1, &uncompressed_string_extract_chars);
-    __ And(temp4, temp4, 0xff);
-    __ Sub(out, temp4.W(), Operand(temp0.W(), UXTB));
-    __ B(&end);
+    // Prioritize the case of compressed strings and calculate such result first.
+    __ Uxtb(temp1, temp4);
+    __ Sub(out, temp1.W(), Operand(temp2.W(), UXTB));
+    __ Tbz(temp3, 0u, &end);  // If actually compressed, we're done.
   }
-  __ Bind(&uncompressed_string_extract_chars);
-  __ And(temp4, temp4, 0xffff);
-  __ Sub(out, temp4.W(), Operand(temp0.W(), UXTH));
-  __ B(&end);
+  __ Uxth(temp4, temp4);
+  __ Sub(out, temp4.W(), Operand(temp2.W(), UXTH));
 
   if (mirror::kUseStringCompression) {
-    vixl::aarch64::Label loop_this_compressed, loop_arg_compressed, find_diff;
+    __ B(&end);
+    __ Bind(&different_compression);
+
+    // Comparison for different compression style.
     const size_t c_char_size = Primitive::ComponentSize(Primitive::kPrimByte);
     DCHECK_EQ(c_char_size, 1u);
-    temp0 = temp0.W();
     temp1 = temp1.W();
-    // Comparison for different compression style.
-    // This part is when THIS is compressed and ARG is not.
-    __ Bind(&different_compression);
-    __ Add(temp0, str, Operand(value_offset));
-    __ Add(temp1, arg, Operand(value_offset));
-    __ Cmp(temp5, Operand(0));
-    __ B(lt, &loop_arg_compressed);
-
-    __ Bind(&loop_this_compressed);
-    __ Ldrb(temp3, MemOperand(temp0.X(), c_char_size, PostIndex));
-    __ Ldrh(temp5, MemOperand(temp1.X(), char_size, PostIndex));
-    __ Cmp(temp3, Operand(temp5));
-    __ B(ne, &find_diff);
-    __ Subs(temp2, temp2, 1);
-    __ B(gt, &loop_this_compressed);
-    __ B(&end);
-
-    // This part is when THIS is not compressed and ARG is.
-    __ Bind(&loop_arg_compressed);
-    __ Ldrh(temp3, MemOperand(temp0.X(), char_size, PostIndex));
-    __ Ldrb(temp5, MemOperand(temp1.X(), c_char_size, PostIndex));
-    __ Cmp(temp3, Operand(temp5));
-    __ B(ne, &find_diff);
-    __ Subs(temp2, temp2, 1);
-    __ B(gt, &loop_arg_compressed);
+    temp2 = temp2.W();
+    temp4 = temp4.W();
+
+    // `temp1` will hold the compressed data pointer, `temp2` the uncompressed data pointer.
+    // Note that flags have been set by the `str` compression flag extraction to `temp3`
+    // before branching to the `different_compression` label.
+    __ Csel(temp1, str, arg, eq);   // Pointer to the compressed string.
+    __ Csel(temp2, str, arg, ne);   // Pointer to the uncompressed string.
+
+    // We want to free up the temp3, currently holding `str` compression flag, for comparison.
+    // So, we move it to the bottom bit of the iteration count `temp0` which we then need to treat
+    // as unsigned. Start by freeing the bit with a LSL and continue further down by a SUB which
+    // will allow `subs temp0, #2; bhi different_compression_loop` to serve as the loop condition.
+    __ Lsl(temp0, temp0, 1u);
+
+    // Adjust temp1 and temp2 from string pointers to data pointers.
+    __ Add(temp1, temp1, Operand(value_offset));
+    __ Add(temp2, temp2, Operand(value_offset));
+
+    // Complete the move of the compression flag.
+    __ Sub(temp0, temp0, Operand(temp3));
+
+    vixl::aarch64::Label different_compression_loop;
+    vixl::aarch64::Label different_compression_diff;
+
+    __ Bind(&different_compression_loop);
+    __ Ldrb(temp4, MemOperand(temp1.X(), c_char_size, PostIndex));
+    __ Ldrh(temp3, MemOperand(temp2.X(), char_size, PostIndex));
+    __ Subs(temp4, temp4, Operand(temp3));
+    __ B(&different_compression_diff, ne);
+    __ Subs(temp0, temp0, 2);
+    __ B(&different_compression_loop, hi);
     __ B(&end);
 
     // Calculate the difference.
-    __ Bind(&find_diff);
-    __ Sub(out, temp3.W(), Operand(temp5.W(), UXTH));
+    __ Bind(&different_compression_diff);
+    __ Tst(temp0, Operand(1));
+    static_assert(static_cast<uint32_t>(mirror::StringCompressionFlag::kCompressed) == 0u,
+                  "Expecting 0=compressed, 1=uncompressed");
+    __ Cneg(out, temp4, ne);
   }
 
   __ Bind(&end);
@@ -1468,7 +1470,7 @@ void IntrinsicCodeGeneratorARM64::VisitStringEquals(HInvoke* invoke) {
   Register temp1 = WRegisterFrom(locations->GetTemp(0));
   Register temp2 = WRegisterFrom(locations->GetTemp(1));
 
-  vixl::aarch64::Label loop, preloop;
+  vixl::aarch64::Label loop;
   vixl::aarch64::Label end;
   vixl::aarch64::Label return_true;
   vixl::aarch64::Label return_false;
@@ -1502,49 +1504,46 @@ void IntrinsicCodeGeneratorARM64::VisitStringEquals(HInvoke* invoke) {
     __ B(&return_false, ne);
   }
 
-  // Load lengths of this and argument strings.
+  // Load `count` fields of this and argument strings.
   __ Ldr(temp, MemOperand(str.X(), count_offset));
   __ Ldr(temp1, MemOperand(arg.X(), count_offset));
-  // Check if lengths are equal, return false if they're not.
+  // Check if `count` fields are equal, return false if they're not.
   // Also compares the compression style, if differs return false.
   __ Cmp(temp, temp1);
   __ B(&return_false, ne);
-  // Return true if both strings are empty.
-  if (mirror::kUseStringCompression) {
-    // Length needs to be masked out first because 0 is treated as compressed.
-    __ Bic(temp, temp, Operand(static_cast<int32_t>(0x80000000)));
-  }
+  // Return true if both strings are empty. Even with string compression `count == 0` means empty.
+  static_assert(static_cast<uint32_t>(mirror::StringCompressionFlag::kCompressed) == 0u,
+                "Expecting 0=compressed, 1=uncompressed");
   __ Cbz(temp, &return_true);
 
-  // Assertions that must hold in order to compare strings 4 characters at a time.
+  // Assertions that must hold in order to compare strings 8 bytes at a time.
   DCHECK_ALIGNED(value_offset, 8);
   static_assert(IsAligned<8>(kObjectAlignment), "String of odd length is not zero padded");
 
   if (mirror::kUseStringCompression) {
-    // If not compressed, directly to fast compare. Else do preprocess on length.
-    __ Cmp(temp1, Operand(0));
-    __ B(&preloop, gt);
-    // Mask out compression flag and adjust length for compressed string (8-bit)
-    // as if it is a 16-bit data, new_length = (length + 1) / 2
-    __ Add(temp, temp, 1);
-    __ Lsr(temp, temp, 1);
+    // For string compression, calculate the number of bytes to compare (not chars).
+    // This could in theory exceed INT32_MAX, so treat temp as unsigned.
+    __ Lsr(temp, temp, 1u);             // Extract length.
+    __ And(temp1, temp1, Operand(1));   // Extract compression flag.
+    __ Lsl(temp, temp, temp1);          // Calculate number of bytes to compare.
   }
 
-  temp1 = temp1.X();
-  temp2 = temp2.X();
-  // Loop to compare strings 4 characters at a time starting at the beginning of the string.
-  // Ok to do this because strings are zero-padded to be 8-byte aligned.
   // Store offset of string value in preparation for comparison loop
-  __ Bind(&preloop);
   __ Mov(temp1, value_offset);
+
+  temp1 = temp1.X();
+  temp2 = temp2.X();
+  // Loop to compare strings 8 bytes at a time starting at the front of the string.
+  // Ok to do this because strings are zero-padded to kObjectAlignment.
   __ Bind(&loop);
   __ Ldr(out, MemOperand(str.X(), temp1));
   __ Ldr(temp2, MemOperand(arg.X(), temp1));
   __ Add(temp1, temp1, Operand(sizeof(uint64_t)));
   __ Cmp(out, temp2);
   __ B(&return_false, ne);
-  __ Sub(temp, temp, Operand(4), SetFlags);
-  __ B(&loop, gt);
+  // With string compression, we have compared 8 bytes, otherwise 4 chars.
+  __ Sub(temp, temp, Operand(mirror::kUseStringCompression ? 8 : 4), SetFlags);
+  __ B(&loop, hi);
 
   // Return true and exit the function.
   // If loop does not result in returning false, we return true.
@@ -1900,10 +1899,6 @@ void IntrinsicLocationsBuilderARM64::VisitStringGetCharsNoCheck(HInvoke* invoke)
   locations->AddTemp(Location::RequiresRegister());
   locations->AddTemp(Location::RequiresRegister());
   locations->AddTemp(Location::RequiresRegister());
-  // Need temporary register for String compression feature.
-  if (mirror::kUseStringCompression) {
-    locations->AddTemp(Location::RequiresRegister());
-  }
 }
 
 void IntrinsicCodeGeneratorARM64::VisitStringGetCharsNoCheck(HInvoke* invoke) {
@@ -1931,10 +1926,6 @@ void IntrinsicCodeGeneratorARM64::VisitStringGetCharsNoCheck(HInvoke* invoke) {
   Register src_ptr = XRegisterFrom(locations->GetTemp(0));
   Register num_chr = XRegisterFrom(locations->GetTemp(1));
   Register tmp1 = XRegisterFrom(locations->GetTemp(2));
-  Register tmp3;
-  if (mirror::kUseStringCompression) {
-    tmp3 = WRegisterFrom(locations->GetTemp(3));
-  }
 
   UseScratchRegisterScope temps(masm);
   Register dst_ptr = temps.AcquireX();
@@ -1957,8 +1948,8 @@ void IntrinsicCodeGeneratorARM64::VisitStringGetCharsNoCheck(HInvoke* invoke) {
     // Location of count in string.
     const uint32_t count_offset = mirror::String::CountOffset().Uint32Value();
     // String's length.
-    __ Ldr(tmp3, MemOperand(srcObj, count_offset));
-    __ Tbnz(tmp3, kWRegSize - 1, &compressed_string_preloop);
+    __ Ldr(tmp2, MemOperand(srcObj, count_offset));
+    __ Tbz(tmp2, 0, &compressed_string_preloop);
   }
   __ Add(src_ptr, src_ptr, Operand(srcBegin, LSL, 1));
 
diff --git a/compiler/optimizing/intrinsics_arm_vixl.cc b/compiler/optimizing/intrinsics_arm_vixl.cc
index 6ff0ca4eab..1450b5a490 100644
--- a/compiler/optimizing/intrinsics_arm_vixl.cc
+++ b/compiler/optimizing/intrinsics_arm_vixl.cc
@@ -1120,7 +1120,6 @@ void IntrinsicLocationsBuilderARMVIXL::VisitStringCompareTo(HInvoke* invoke) {
   // Need temporary registers for String compression's feature.
   if (mirror::kUseStringCompression) {
     locations->AddTemp(Location::RequiresRegister());
-    locations->AddTemp(Location::RequiresRegister());
   }
   locations->SetOut(Location::RequiresRegister(), Location::kOutputOverlap);
 }
@@ -1136,10 +1135,9 @@ void IntrinsicCodeGeneratorARMVIXL::VisitStringCompareTo(HInvoke* invoke) {
   vixl32::Register temp0 = RegisterFrom(locations->GetTemp(0));
   vixl32::Register temp1 = RegisterFrom(locations->GetTemp(1));
   vixl32::Register temp2 = RegisterFrom(locations->GetTemp(2));
-  vixl32::Register temp3, temp4;
+  vixl32::Register temp3;
   if (mirror::kUseStringCompression) {
     temp3 = RegisterFrom(locations->GetTemp(3));
-    temp4 = RegisterFrom(locations->GetTemp(4));
   }
 
   vixl32::Label loop;
@@ -1167,23 +1165,20 @@ void IntrinsicCodeGeneratorARMVIXL::VisitStringCompareTo(HInvoke* invoke) {
   __ Subs(out, str, arg);
   __ B(eq, &end);
 
-  UseScratchRegisterScope temps(assembler->GetVIXLAssembler());
-  vixl32::Register temp_reg = temps.Acquire();
-
   if (mirror::kUseStringCompression) {
-    // Load lengths of this and argument strings.
+    // Load `count` fields of this and argument strings.
     __ Ldr(temp3, MemOperand(str, count_offset));
-    __ Ldr(temp4, MemOperand(arg, count_offset));
-    // Clean out compression flag from lengths.
-    __ Bic(temp0, temp3, 0x80000000);
-    __ Bic(temp_reg, temp4, 0x80000000);
+    __ Ldr(temp2, MemOperand(arg, count_offset));
+    // Extract lengths from the `count` fields.
+    __ Lsr(temp0, temp3, 1u);
+    __ Lsr(temp1, temp2, 1u);
   } else {
     // Load lengths of this and argument strings.
     __ Ldr(temp0, MemOperand(str, count_offset));
-    __ Ldr(temp_reg, MemOperand(arg, count_offset));
+    __ Ldr(temp1, MemOperand(arg, count_offset));
   }
   // out = length diff.
-  __ Subs(out, temp0, temp_reg);
+  __ Subs(out, temp0, temp1);
   // temp0 = min(len(str), len(arg)).
 
   {
@@ -1192,33 +1187,32 @@ void IntrinsicCodeGeneratorARMVIXL::VisitStringCompareTo(HInvoke* invoke) {
                                CodeBufferCheckScope::kMaximumSize);
 
     __ it(gt);
-    __ mov(gt, temp0, temp_reg);
+    __ mov(gt, temp0, temp1);
   }
 
-  temps.Release(temp_reg);
   // Shorter string is empty?
   __ Cbz(temp0, &end);
 
   if (mirror::kUseStringCompression) {
     // Check if both strings using same compression style to use this comparison loop.
-    __ Eors(temp3, temp3, temp4);
-    __ B(mi, &different_compression);
-  }
-  // Store offset of string value in preparation for comparison loop.
-  __ Mov(temp1, value_offset);
-  if (mirror::kUseStringCompression) {
+    __ Eors(temp2, temp2, temp3);
+    __ Lsrs(temp2, temp2, 1u);
+    __ B(cs, &different_compression);
     // For string compression, calculate the number of bytes to compare (not chars).
     // This could in theory exceed INT32_MAX, so treat temp0 as unsigned.
-    __ Cmp(temp4, 0);
+    __ Lsls(temp3, temp3, 31u);  // Extract purely the compression flag.
 
     AssemblerAccurateScope aas(assembler->GetVIXLAssembler(),
                                2 * kMaxInstructionSizeInBytes,
                                CodeBufferCheckScope::kMaximumSize);
 
-    __ it(ge);
-    __ add(ge, temp0, temp0, temp0);
+    __ it(ne);
+    __ add(ne, temp0, temp0, temp0);
   }
 
+  // Store offset of string value in preparation for comparison loop.
+  __ Mov(temp1, value_offset);
+
   // Assertions that must hold in order to compare multiple characters at a time.
   CHECK_ALIGNED(value_offset, 8);
   static_assert(IsAligned<8>(kObjectAlignment),
@@ -1227,10 +1221,12 @@ void IntrinsicCodeGeneratorARMVIXL::VisitStringCompareTo(HInvoke* invoke) {
   const size_t char_size = Primitive::ComponentSize(Primitive::kPrimChar);
   DCHECK_EQ(char_size, 2u);
 
+  UseScratchRegisterScope temps(assembler->GetVIXLAssembler());
+
   vixl32::Label find_char_diff_2nd_cmp;
   // Unrolled loop comparing 4x16-bit chars per iteration (ok because of string data alignment).
   __ Bind(&loop);
-  temp_reg = temps.Acquire();
+  vixl32::Register temp_reg = temps.Acquire();
   __ Ldr(temp_reg, MemOperand(str, temp1));
   __ Ldr(temp2, MemOperand(arg, temp1));
   __ Cmp(temp_reg, temp2);
@@ -1279,72 +1275,92 @@ void IntrinsicCodeGeneratorARMVIXL::VisitStringCompareTo(HInvoke* invoke) {
   // The comparison is unsigned for string compression, otherwise signed.
   __ Cmp(temp0, Operand(temp1, vixl32::LSR, (mirror::kUseStringCompression ? 3 : 4)));
   __ B((mirror::kUseStringCompression ? ls : le), &end);
+
   // Extract the characters and calculate the difference.
-  vixl32::Label uncompressed_string, continue_process;
   if (mirror::kUseStringCompression) {
-    __ Cmp(temp4, 0);
-    __ B(ge, &uncompressed_string);
-    __ Bic(temp1, temp1, 0x7);
-    __ B(&continue_process);
+    // For compressed strings we need to clear 0x7 from temp1, for uncompressed we need to clear
+    // 0xf. We also need to prepare the character extraction mask `uncompressed ? 0xffffu : 0xffu`.
+    // The compression flag is now in the highest bit of temp3, so let's play some tricks.
+    __ orr(temp3, temp3, 0xffu << 23);                  // uncompressed ? 0xff800000u : 0x7ff80000u
+    __ bic(temp1, temp1, Operand(temp3, vixl32::LSR, 31 - 3));  // &= ~(uncompressed ? 0xfu : 0x7u)
+    __ Asr(temp3, temp3, 7u);                           // uncompressed ? 0xffff0000u : 0xff0000u.
+    __ Lsr(temp2, temp2, temp1);                        // Extract second character.
+    __ Lsr(temp3, temp3, 16u);                          // uncompressed ? 0xffffu : 0xffu
+    __ Lsr(out, temp_reg, temp1);                       // Extract first character.
+    __ and_(temp2, temp2, temp3);
+    __ and_(out, out, temp3);
+  } else {
+    __ bic(temp1, temp1, 0xf);
+    __ Lsr(temp2, temp2, temp1);
+    __ Lsr(out, temp_reg, temp1);
+    __ movt(temp2, 0);
+    __ movt(out, 0);
   }
-  __ Bind(&uncompressed_string);
-  __ Bic(temp1, temp1, 0xf);
-  __ Bind(&continue_process);
 
-  __ Lsr(temp2, temp2, temp1);
-  __ Lsr(temp_reg, temp_reg, temp1);
-  vixl32::Label calculate_difference, uncompressed_string_extract_chars;
-  if (mirror::kUseStringCompression) {
-    __ Cmp(temp4, 0);
-    __ B(ge, &uncompressed_string_extract_chars);
-    __ Ubfx(temp2, temp2, 0, 8);
-    __ Ubfx(temp_reg, temp_reg, 0, 8);
-    __ B(&calculate_difference);
-  }
-  __ Bind(&uncompressed_string_extract_chars);
-  __ Movt(temp2, 0);
-  __ Movt(temp_reg, 0);
-  __ Bind(&calculate_difference);
-  __ Sub(out, temp_reg, temp2);
+  __ Sub(out, out, temp2);
   temps.Release(temp_reg);
-  __ B(&end);
 
   if (mirror::kUseStringCompression) {
+    __ B(&end);
+    __ Bind(&different_compression);
+
+    // Comparison for different compression style.
     const size_t c_char_size = Primitive::ComponentSize(Primitive::kPrimByte);
     DCHECK_EQ(c_char_size, 1u);
-    vixl32::Label loop_arg_compressed, loop_this_compressed, find_diff;
-    // Comparison for different compression style.
-    // This part is when THIS is compressed and ARG is not.
-    __ Bind(&different_compression);
-    __ Add(temp2, str, value_offset);
-    __ Add(temp3, arg, value_offset);
-    __ Cmp(temp4, 0);
-    __ B(lt, &loop_arg_compressed);
 
-    __ Bind(&loop_this_compressed);
-    temp_reg = temps.Acquire();
-    __ Ldrb(temp_reg, MemOperand(temp2, c_char_size, PostIndex));
-    __ Ldrh(temp4, MemOperand(temp3, char_size, PostIndex));
-    __ Cmp(temp_reg, temp4);
-    __ B(ne, &find_diff);
-    __ Subs(temp0, temp0, 1);
-    __ B(gt, &loop_this_compressed);
-    __ B(&end);
+    // We want to free up the temp3, currently holding `str.count`, for comparison.
+    // So, we move it to the bottom bit of the iteration count `temp0` which we tnen
+    // need to treat as unsigned. Start by freeing the bit with an ADD and continue
+    // further down by a LSRS+SBC which will flip the meaning of the flag but allow
+    // `subs temp0, #2; bhi different_compression_loop` to serve as the loop condition.
+    __ add(temp0, temp0, temp0);              // Unlike LSL, this ADD is always 16-bit.
+    // `temp1` will hold the compressed data pointer, `temp2` the uncompressed data pointer.
+    __ mov(temp1, str);
+    __ mov(temp2, arg);
+    __ Lsrs(temp3, temp3, 1u);                // Continue the move of the compression flag.
+    {
+      AssemblerAccurateScope aas(assembler->GetVIXLAssembler(),
+                                 3 * kMaxInstructionSizeInBytes,
+                                 CodeBufferCheckScope::kMaximumSize);
+      __ itt(cs);                             // Interleave with selection of temp1 and temp2.
+      __ mov(cs, temp1, arg);                 // Preserves flags.
+      __ mov(cs, temp2, str);                 // Preserves flags.
+    }
+    __ sbc(temp0, temp0, 0);                  // Complete the move of the compression flag.
+
+    // Adjust temp1 and temp2 from string pointers to data pointers.
+    __ add(temp1, temp1, value_offset);
+    __ add(temp2, temp2, value_offset);
 
-    // This part is when THIS is not compressed and ARG is.
-    __ Bind(&loop_arg_compressed);
-    __ Ldrh(temp_reg, MemOperand(temp2, char_size, PostIndex));
-    __ Ldrb(temp4, MemOperand(temp3, c_char_size, PostIndex));
-    __ Cmp(temp_reg, temp4);
-    __ B(ne, &find_diff);
-    __ Subs(temp0, temp0, 1);
-    __ B(gt, &loop_arg_compressed);
+    vixl32::Label different_compression_loop;
+    vixl32::Label different_compression_diff;
+
+    // Main loop for different compression.
+    temp_reg = temps.Acquire();
+    __ Bind(&different_compression_loop);
+    __ Ldrb(temp_reg, MemOperand(temp1, c_char_size, PostIndex));
+    __ Ldrh(temp3, MemOperand(temp2, char_size, PostIndex));
+    __ cmp(temp_reg, temp3);
+    __ B(ne, &different_compression_diff);
+    __ Subs(temp0, temp0, 2);
+    __ B(hi, &different_compression_loop);
     __ B(&end);
 
     // Calculate the difference.
-    __ Bind(&find_diff);
-    __ Sub(out, temp_reg, temp4);
+    __ Bind(&different_compression_diff);
+    __ Sub(out, temp_reg, temp3);
     temps.Release(temp_reg);
+    // Flip the difference if the `arg` is compressed.
+    // `temp0` contains inverted `str` compression flag, i.e the same as `arg` compression flag.
+    __ Lsrs(temp0, temp0, 1u);
+    static_assert(static_cast<uint32_t>(mirror::StringCompressionFlag::kCompressed) == 0u,
+                  "Expecting 0=compressed, 1=uncompressed");
+
+    AssemblerAccurateScope aas(assembler->GetVIXLAssembler(),
+                               2 * kMaxInstructionSizeInBytes,
+                               CodeBufferCheckScope::kMaximumSize);
+    __ it(cc);
+    __ rsb(cc, out, out, 0);
   }
 
   __ Bind(&end);
@@ -1382,7 +1398,7 @@ void IntrinsicCodeGeneratorARMVIXL::VisitStringEquals(HInvoke* invoke) {
   vixl32::Register temp1 = RegisterFrom(locations->GetTemp(1));
   vixl32::Register temp2 = RegisterFrom(locations->GetTemp(2));
 
-  vixl32::Label loop, preloop;
+  vixl32::Label loop;
   vixl32::Label end;
   vixl32::Label return_true;
   vixl32::Label return_false;
@@ -1401,6 +1417,10 @@ void IntrinsicCodeGeneratorARMVIXL::VisitStringEquals(HInvoke* invoke) {
     __ Cbz(arg, &return_false);
   }
 
+  // Reference equality check, return true if same reference.
+  __ Cmp(str, arg);
+  __ B(eq, &return_true);
+
   if (!optimizations.GetArgumentIsString()) {
     // Instanceof check for the argument by comparing class fields.
     // All string objects must have the same type since String cannot be subclassed.
@@ -1412,48 +1432,47 @@ void IntrinsicCodeGeneratorARMVIXL::VisitStringEquals(HInvoke* invoke) {
     __ B(ne, &return_false);
   }
 
-  // Load lengths of this and argument strings.
+  // Load `count` fields of this and argument strings.
   __ Ldr(temp, MemOperand(str, count_offset));
   __ Ldr(temp1, MemOperand(arg, count_offset));
-  // Check if lengths are equal, return false if they're not.
+  // Check if `count` fields are equal, return false if they're not.
   // Also compares the compression style, if differs return false.
   __ Cmp(temp, temp1);
   __ B(ne, &return_false);
-  // Return true if both strings are empty.
-  if (mirror::kUseStringCompression) {
-    // Length needs to be masked out first because 0 is treated as compressed.
-    __ Bic(temp, temp, 0x80000000);
-  }
+  // Return true if both strings are empty. Even with string compression `count == 0` means empty.
+  static_assert(static_cast<uint32_t>(mirror::StringCompressionFlag::kCompressed) == 0u,
+                "Expecting 0=compressed, 1=uncompressed");
   __ Cbz(temp, &return_true);
-  // Reference equality check, return true if same reference.
-  __ Cmp(str, arg);
-  __ B(eq, &return_true);
 
-  // Assertions that must hold in order to compare strings 2 characters at a time.
+  // Assertions that must hold in order to compare strings 4 bytes at a time.
   DCHECK_ALIGNED(value_offset, 4);
   static_assert(IsAligned<4>(kObjectAlignment), "String data must be aligned for fast compare.");
 
   if (mirror::kUseStringCompression) {
-    // If not compressed, directly to fast compare. Else do preprocess on length.
-    __ Cmp(temp1, 0);
-    __ B(gt, &preloop);
-    // Mask out compression flag and adjust length for compressed string (8-bit)
-    // as if it is a 16-bit data, new_length = (length + 1) / 2.
-    __ Add(temp, temp, 1);
-    __ Lsr(temp, temp, 1);
-    __ Bind(&preloop);
+    // For string compression, calculate the number of bytes to compare (not chars).
+    // This could in theory exceed INT32_MAX, so treat temp as unsigned.
+    __ Lsrs(temp, temp, 1u);                        // Extract length and check compression flag.
+    AssemblerAccurateScope aas(assembler->GetVIXLAssembler(),
+                               2 * kMaxInstructionSizeInBytes,
+                               CodeBufferCheckScope::kMaximumSize);
+    __ it(cs);                                      // If uncompressed,
+    __ add(cs, temp, temp, temp);                   //   double the byte count.
   }
-  // Loop to compare strings 2 characters at a time starting at the front of the string.
-  // Ok to do this because strings with an odd length are zero-padded.
+
+  // Store offset of string value in preparation for comparison loop.
   __ Mov(temp1, value_offset);
+
+  // Loop to compare strings 4 bytes at a time starting at the front of the string.
+  // Ok to do this because strings are zero-padded to kObjectAlignment.
   __ Bind(&loop);
   __ Ldr(out, MemOperand(str, temp1));
   __ Ldr(temp2, MemOperand(arg, temp1));
+  __ Add(temp1, temp1, sizeof(uint32_t));
   __ Cmp(out, temp2);
   __ B(ne, &return_false);
-  __ Add(temp1, temp1, sizeof(uint32_t));
-  __ Subs(temp, temp, sizeof(uint32_t) / sizeof(uint16_t));
-  __ B(gt, &loop);
+  // With string compression, we have compared 4 bytes, otherwise 2 chars.
+  __ Subs(temp, temp, mirror::kUseStringCompression ? 4 : 2);
+  __ B(hi, &loop);
 
   // Return true and exit the function.
   // If loop does not result in returning false, we return true.
@@ -2547,9 +2566,9 @@ void IntrinsicCodeGeneratorARMVIXL::VisitStringGetCharsNoCheck(HInvoke* invoke)
     temp = temps.Acquire();
     // String's length.
     __ Ldr(temp, MemOperand(srcObj, count_offset));
-    __ Cmp(temp, 0);
+    __ Tst(temp, 1);
     temps.Release(temp);
-    __ B(lt, &compressed_string_preloop);
+    __ B(eq, &compressed_string_preloop);
   }
   __ Add(src_ptr, src_ptr, Operand(srcBegin, vixl32::LSL, 1));
 
@@ -2588,9 +2607,10 @@ void IntrinsicCodeGeneratorARMVIXL::VisitStringGetCharsNoCheck(HInvoke* invoke)
   __ Strh(temp, MemOperand(dst_ptr, char_size, PostIndex));
   temps.Release(temp);
   __ B(gt, &remainder);
-  __ B(&done);
 
   if (mirror::kUseStringCompression) {
+    __ B(&done);
+
     const size_t c_char_size = Primitive::ComponentSize(Primitive::kPrimByte);
     DCHECK_EQ(c_char_size, 1u);
     // Copy loop for compressed src, copying 1 character (8-bit) to (16-bit) at a time.
diff --git a/compiler/optimizing/intrinsics_x86.cc b/compiler/optimizing/intrinsics_x86.cc
index 43682c5633..80b4df62a9 100644
--- a/compiler/optimizing/intrinsics_x86.cc
+++ b/compiler/optimizing/intrinsics_x86.cc
@@ -1408,21 +1408,22 @@ void IntrinsicCodeGeneratorX86::VisitStringEquals(HInvoke* invoke) {
   // compression style is decided on alloc.
   __ cmpl(ecx, Address(arg, count_offset));
   __ j(kNotEqual, &return_false);
+  // Return true if strings are empty. Even with string compression `count == 0` means empty.
+  static_assert(static_cast<uint32_t>(mirror::StringCompressionFlag::kCompressed) == 0u,
+                "Expecting 0=compressed, 1=uncompressed");
+  __ jecxz(&return_true);
 
   if (mirror::kUseStringCompression) {
     NearLabel string_uncompressed;
-    // Differ cases into both compressed or both uncompressed. Different compression style
-    // is cut above.
-    __ cmpl(ecx, Immediate(0));
-    __ j(kGreaterEqual, &string_uncompressed);
+    // Extract length and differentiate between both compressed or both uncompressed.
+    // Different compression style is cut above.
+    __ shrl(ecx, Immediate(1));
+    __ j(kCarrySet, &string_uncompressed);
     // Divide string length by 2, rounding up, and continue as if uncompressed.
-    // Merge clearing the compression flag (+0x80000000) with +1 for rounding.
-    __ addl(ecx, Immediate(0x80000001));
+    __ addl(ecx, Immediate(1));
     __ shrl(ecx, Immediate(1));
     __ Bind(&string_uncompressed);
   }
-  // Return true if strings are empty.
-  __ jecxz(&return_true);
   // Load starting addresses of string values into ESI/EDI as required for repe_cmpsl instruction.
   __ leal(esi, Address(str, value_offset));
   __ leal(edi, Address(arg, value_offset));
@@ -1535,21 +1536,24 @@ static void GenerateStringIndexOf(HInvoke* invoke,
   // Location of count within the String object.
   int32_t count_offset = mirror::String::CountOffset().Int32Value();
 
-  // Load string length, i.e., the count field of the string.
+  // Load the count field of the string containing the length and compression flag.
   __ movl(string_length, Address(string_obj, count_offset));
-  if (mirror::kUseStringCompression) {
-    string_length_flagged = locations->GetTemp(2).AsRegister<Register>();
-    __ movl(string_length_flagged, string_length);
-    // Mask out first bit used as compression flag.
-    __ andl(string_length, Immediate(INT32_MAX));
-  }
 
-  // Do a zero-length check.
+  // Do a zero-length check. Even with string compression `count == 0` means empty.
+  static_assert(static_cast<uint32_t>(mirror::StringCompressionFlag::kCompressed) == 0u,
+                "Expecting 0=compressed, 1=uncompressed");
   // TODO: Support jecxz.
   NearLabel not_found_label;
   __ testl(string_length, string_length);
   __ j(kEqual, &not_found_label);
 
+  if (mirror::kUseStringCompression) {
+    string_length_flagged = locations->GetTemp(2).AsRegister<Register>();
+    __ movl(string_length_flagged, string_length);
+    // Extract the length and shift out the least significant bit used as compression flag.
+    __ shrl(string_length, Immediate(1));
+  }
+
   if (start_at_zero) {
     // Number of chars to scan is the same as the string length.
     __ movl(counter, string_length);
@@ -1570,8 +1574,8 @@ static void GenerateStringIndexOf(HInvoke* invoke,
 
     if (mirror::kUseStringCompression) {
       NearLabel modify_counter, offset_uncompressed_label;
-      __ cmpl(string_length_flagged, Immediate(0));
-      __ j(kGreaterEqual, &offset_uncompressed_label);
+      __ testl(string_length_flagged, Immediate(1));
+      __ j(kNotZero, &offset_uncompressed_label);
       // Move to the start of the string: string_obj + value_offset + start_index.
       __ leal(string_obj, Address(string_obj, counter, ScaleFactor::TIMES_1, value_offset));
       __ jmp(&modify_counter);
@@ -1593,8 +1597,8 @@ static void GenerateStringIndexOf(HInvoke* invoke,
   if (mirror::kUseStringCompression) {
     NearLabel uncompressed_string_comparison;
     NearLabel comparison_done;
-    __ cmpl(string_length_flagged, Immediate(0));
-    __ j(kGreater, &uncompressed_string_comparison);
+    __ testl(string_length_flagged, Immediate(1));
+    __ j(kNotZero, &uncompressed_string_comparison);
 
     // Check if EAX (search_value) is ASCII.
     __ cmpl(search_value, Immediate(127));
@@ -1787,8 +1791,10 @@ void IntrinsicCodeGeneratorX86::VisitStringGetCharsNoCheck(HInvoke* invoke) {
     __ cfi().AdjustCFAOffset(stack_adjust);
 
     NearLabel copy_loop, copy_uncompressed;
-    __ cmpl(Address(obj, count_offset), Immediate(0));
-    __ j(kGreaterEqual, &copy_uncompressed);
+    __ testl(Address(obj, count_offset), Immediate(1));
+    static_assert(static_cast<uint32_t>(mirror::StringCompressionFlag::kCompressed) == 0u,
+                  "Expecting 0=compressed, 1=uncompressed");
+    __ j(kNotZero, &copy_uncompressed);
     // Compute the address of the source string by adding the number of chars from
     // the source beginning to the value offset of a string.
     __ leal(ESI, CodeGeneratorX86::ArrayAddress(obj, srcBegin, TIMES_1, value_offset));
diff --git a/compiler/optimizing/intrinsics_x86_64.cc b/compiler/optimizing/intrinsics_x86_64.cc
index de2606c327..7ecf4ba6c4 100644
--- a/compiler/optimizing/intrinsics_x86_64.cc
+++ b/compiler/optimizing/intrinsics_x86_64.cc
@@ -1574,20 +1574,23 @@ void IntrinsicCodeGeneratorX86_64::VisitStringEquals(HInvoke* invoke) {
   // compression style is decided on alloc.
   __ cmpl(rcx, Address(arg, count_offset));
   __ j(kNotEqual, &return_false);
+  // Return true if both strings are empty. Even with string compression `count == 0` means empty.
+  static_assert(static_cast<uint32_t>(mirror::StringCompressionFlag::kCompressed) == 0u,
+                "Expecting 0=compressed, 1=uncompressed");
+  __ jrcxz(&return_true);
 
   if (mirror::kUseStringCompression) {
     NearLabel string_uncompressed;
-    // Both string are compressed.
-    __ cmpl(rcx, Immediate(0));
-    __ j(kGreaterEqual, &string_uncompressed);
+    // Extract length and differentiate between both compressed or both uncompressed.
+    // Different compression style is cut above.
+    __ shrl(rcx, Immediate(1));
+    __ j(kCarrySet, &string_uncompressed);
     // Divide string length by 2, rounding up, and continue as if uncompressed.
     // Merge clearing the compression flag with +1 for rounding.
-    __ addl(rcx, Immediate(static_cast<int32_t>(0x80000001)));
+    __ addl(rcx, Immediate(1));
     __ shrl(rcx, Immediate(1));
     __ Bind(&string_uncompressed);
   }
-  // Return true if both strings are empty.
-  __ jrcxz(&return_true);
   // Load starting addresses of string values into RSI/RDI as required for repe_cmpsq instruction.
   __ leal(rsi, Address(str, value_offset));
   __ leal(rdi, Address(arg, value_offset));
@@ -1694,21 +1697,22 @@ static void GenerateStringIndexOf(HInvoke* invoke,
   // Location of count within the String object.
   int32_t count_offset = mirror::String::CountOffset().Int32Value();
 
-  // Load string length, i.e., the count field of the string.
+  // Load the count field of the string containing the length and compression flag.
   __ movl(string_length, Address(string_obj, count_offset));
-  if (mirror::kUseStringCompression) {
-    // Use TMP to keep string_length_flagged.
-    __ movl(CpuRegister(TMP), string_length);
-    // Mask out first bit used as compression flag.
-    __ andl(string_length, Immediate(INT32_MAX));
-  }
 
-  // Do a length check.
+  // Do a zero-length check. Even with string compression `count == 0` means empty.
   // TODO: Support jecxz.
   NearLabel not_found_label;
   __ testl(string_length, string_length);
   __ j(kEqual, &not_found_label);
 
+  if (mirror::kUseStringCompression) {
+    // Use TMP to keep string_length_flagged.
+    __ movl(CpuRegister(TMP), string_length);
+    // Mask out first bit used as compression flag.
+    __ shrl(string_length, Immediate(1));
+  }
+
   if (start_at_zero) {
     // Number of chars to scan is the same as the string length.
     __ movl(counter, string_length);
@@ -1728,8 +1732,8 @@ static void GenerateStringIndexOf(HInvoke* invoke,
 
     if (mirror::kUseStringCompression) {
       NearLabel modify_counter, offset_uncompressed_label;
-      __ cmpl(CpuRegister(TMP), Immediate(0));
-      __ j(kGreaterEqual, &offset_uncompressed_label);
+      __ testl(CpuRegister(TMP), Immediate(1));
+      __ j(kNotZero, &offset_uncompressed_label);
       __ leaq(string_obj, Address(string_obj, counter, ScaleFactor::TIMES_1, value_offset));
       __ jmp(&modify_counter);
       // Move to the start of the string: string_obj + value_offset + 2 * start_index.
@@ -1747,8 +1751,8 @@ static void GenerateStringIndexOf(HInvoke* invoke,
   if (mirror::kUseStringCompression) {
     NearLabel uncompressed_string_comparison;
     NearLabel comparison_done;
-    __ cmpl(CpuRegister(TMP), Immediate(0));
-    __ j(kGreater, &uncompressed_string_comparison);
+    __ testl(CpuRegister(TMP), Immediate(1));
+    __ j(kNotZero, &uncompressed_string_comparison);
     // Check if RAX (search_value) is ASCII.
     __ cmpl(search_value, Immediate(127));
     __ j(kGreater, &not_found_label);
@@ -1931,8 +1935,10 @@ void IntrinsicCodeGeneratorX86_64::VisitStringGetCharsNoCheck(HInvoke* invoke) {
     // Location of count in string.
     const uint32_t count_offset = mirror::String::CountOffset().Uint32Value();
 
-    __ cmpl(Address(obj, count_offset), Immediate(0));
-    __ j(kGreaterEqual, &copy_uncompressed);
+    __ testl(Address(obj, count_offset), Immediate(1));
+    static_assert(static_cast<uint32_t>(mirror::StringCompressionFlag::kCompressed) == 0u,
+                  "Expecting 0=compressed, 1=uncompressed");
+    __ j(kNotZero, &copy_uncompressed);
     // Compute the address of the source string by adding the number of chars from
     // the source beginning to the value offset of a string.
     __ leaq(CpuRegister(RSI),
diff --git a/runtime/arch/arm/quick_entrypoints_arm.S b/runtime/arch/arm/quick_entrypoints_arm.S
index 0135260de8..b76050ce18 100644
--- a/runtime/arch/arm/quick_entrypoints_arm.S
+++ b/runtime/arch/arm/quick_entrypoints_arm.S
@@ -1768,12 +1768,15 @@ ENTRY art_quick_indexof
     .cfi_rel_offset r10, 4
     .cfi_rel_offset r11, 8
     .cfi_rel_offset lr, 12
+#if (STRING_COMPRESSION_FEATURE)
+    ldr   r4, [r0, #MIRROR_STRING_COUNT_OFFSET]
+#else
     ldr   r3, [r0, #MIRROR_STRING_COUNT_OFFSET]
+#endif
     add   r0, #MIRROR_STRING_VALUE_OFFSET
 #if (STRING_COMPRESSION_FEATURE)
     /* r4 count (with flag) and r3 holds actual length */
-    mov   r4, r3
-    bic   r3, #2147483648
+    lsr   r3, r4, #1
 #endif
     /* Clamp start to [0..count] */
     cmp   r2, #0
@@ -1788,8 +1791,8 @@ ENTRY art_quick_indexof
 
     /* Build pointer to start of data to compare and pre-bias */
 #if (STRING_COMPRESSION_FEATURE)
-    cmp   r4, #0
-    blt   .Lstring_indexof_compressed
+    lsrs  r4, r4, #1
+    bcc   .Lstring_indexof_compressed
 #endif
     add   r0, r0, r2, lsl #1
     sub   r0, #2
diff --git a/runtime/arch/arm64/quick_entrypoints_arm64.S b/runtime/arch/arm64/quick_entrypoints_arm64.S
index d806715ec9..74643f70a0 100644
--- a/runtime/arch/arm64/quick_entrypoints_arm64.S
+++ b/runtime/arch/arm64/quick_entrypoints_arm64.S
@@ -2402,12 +2402,15 @@ END art_quick_deoptimize_from_compiled_code
      *    w2:   Starting offset in string data
      */
 ENTRY art_quick_indexof
+#if (STRING_COMPRESSION_FEATURE)
+    ldr   w4, [x0, #MIRROR_STRING_COUNT_OFFSET]
+#else
     ldr   w3, [x0, #MIRROR_STRING_COUNT_OFFSET]
+#endif
     add   x0, x0, #MIRROR_STRING_VALUE_OFFSET
 #if (STRING_COMPRESSION_FEATURE)
     /* w4 holds count (with flag) and w3 holds actual length */
-    mov   w4, w3
-    and   w3, w3, #2147483647
+    lsr   w3, w4, #1
 #endif
     /* Clamp start to [0..count] */
     cmp   w2, #0
@@ -2419,7 +2422,7 @@ ENTRY art_quick_indexof
     mov   x5, x0
 
 #if (STRING_COMPRESSION_FEATURE)
-    tbnz  w4, #31, .Lstring_indexof_compressed
+    tbz   w4, #0, .Lstring_indexof_compressed
 #endif
     /* Build pointer to start of data to compare and pre-bias */
     add   x0, x0, x2, lsl #1
diff --git a/runtime/arch/x86/quick_entrypoints_x86.S b/runtime/arch/x86/quick_entrypoints_x86.S
index 98739d3cc8..0f1efcee69 100644
--- a/runtime/arch/x86/quick_entrypoints_x86.S
+++ b/runtime/arch/x86/quick_entrypoints_x86.S
@@ -2035,15 +2035,14 @@ DEFINE_FUNCTION art_quick_string_compareto
     lea MIRROR_STRING_VALUE_OFFSET(%ecx), %edi
 #if (STRING_COMPRESSION_FEATURE)
     /* Differ cases */
-    cmpl    LITERAL(0), %edx
-    jl      .Lstring_compareto_this_is_compressed
-    cmpl    LITERAL(0), %ebx
-    jl      .Lstring_compareto_that_is_compressed
+    shrl    LITERAL(1), %edx
+    jnc     .Lstring_compareto_this_is_compressed
+    shrl    LITERAL(1), %ebx
+    jnc     .Lstring_compareto_that_is_compressed
     jmp     .Lstring_compareto_both_not_compressed
 .Lstring_compareto_this_is_compressed:
-    andl    LITERAL(0x7FFFFFFF), %edx
-    cmpl    LITERAL(0), %ebx
-    jl      .Lstring_compareto_both_compressed
+    shrl    LITERAL(1), %ebx
+    jnc     .Lstring_compareto_both_compressed
     /* If (this->IsCompressed() && that->IsCompressed() == false) */
     mov     %edx, %eax
     subl    %ebx, %eax
@@ -2061,7 +2060,6 @@ DEFINE_FUNCTION art_quick_string_compareto
     cmovne  %edx, %eax                        // return eax = *(this_cur_char) - *(that_cur_char)
     jmp     .Lstring_compareto_return
 .Lstring_compareto_that_is_compressed:
-    andl    LITERAL(0x7FFFFFFF), %ebx
     mov     %edx, %eax
     subl    %ebx, %eax
     mov     %edx, %ecx
@@ -2078,7 +2076,6 @@ DEFINE_FUNCTION art_quick_string_compareto
     cmovne  %edx, %eax
     jmp     .Lstring_compareto_return         // return eax = *(this_cur_char) - *(that_cur_char)
 .Lstring_compareto_both_compressed:
-    andl    LITERAL(0x7FFFFFFF), %ebx
     /* Calculate min length and count diff */
     mov     %edx, %ecx
     mov     %edx, %eax
diff --git a/runtime/arch/x86_64/quick_entrypoints_x86_64.S b/runtime/arch/x86_64/quick_entrypoints_x86_64.S
index 185e55e114..0d0ab93b8a 100644
--- a/runtime/arch/x86_64/quick_entrypoints_x86_64.S
+++ b/runtime/arch/x86_64/quick_entrypoints_x86_64.S
@@ -2142,15 +2142,14 @@ DEFINE_FUNCTION art_quick_string_compareto
     leal MIRROR_STRING_VALUE_OFFSET(%esi), %esi
 #if (STRING_COMPRESSION_FEATURE)
     /* Differ cases */
-    cmpl LITERAL(0), %r8d
-    jl      .Lstring_compareto_this_is_compressed
-    cmpl    LITERAL(0), %r9d
-    jl      .Lstring_compareto_that_is_compressed
+    shrl    LITERAL(1), %r8d
+    jnc     .Lstring_compareto_this_is_compressed
+    shrl    LITERAL(1), %r9d
+    jnc     .Lstring_compareto_that_is_compressed
     jmp     .Lstring_compareto_both_not_compressed
 .Lstring_compareto_this_is_compressed:
-    andl    LITERAL(0x7FFFFFFF), %r8d
-    cmpl    LITERAL(0), %r9d
-    jl      .Lstring_compareto_both_compressed
+    shrl    LITERAL(1), %r9d
+    jnc     .Lstring_compareto_both_compressed
     /* Comparison this (8-bit) and that (16-bit) */
     mov     %r8d, %eax
     subl    %r9d, %eax
@@ -2169,7 +2168,6 @@ DEFINE_FUNCTION art_quick_string_compareto
 .Lstring_compareto_keep_length1:
     ret
 .Lstring_compareto_that_is_compressed:
-    andl    LITERAL(0x7FFFFFFF), %r9d
     movl    %r8d, %eax
     subl    %r9d, %eax
     mov     %r8d, %ecx
@@ -2187,7 +2185,6 @@ DEFINE_FUNCTION art_quick_string_compareto
 .Lstring_compareto_keep_length2:
     ret
 .Lstring_compareto_both_compressed:
-    andl    LITERAL(0x7FFFFFFF), %r9d
     /* Calculate min length and count diff */
     movl    %r8d, %ecx
     movl    %r8d, %eax
diff --git a/runtime/interpreter/unstarted_runtime.cc b/runtime/interpreter/unstarted_runtime.cc
index 6bf7e15f86..2257fd6395 100644
--- a/runtime/interpreter/unstarted_runtime.cc
+++ b/runtime/interpreter/unstarted_runtime.cc
@@ -1097,10 +1097,12 @@ void UnstartedRuntime::UnstartedStringGetCharsNoCheck(
     return;
   }
   DCHECK_GE(start, 0);
-  DCHECK_GE(end, string->GetLength());
+  DCHECK_LE(start, end);
+  DCHECK_LE(end, string->GetLength());
   StackHandleScope<1> hs(self);
   Handle<mirror::CharArray> h_char_array(
       hs.NewHandle(shadow_frame->GetVRegReference(arg_offset + 3)->AsCharArray()));
+  DCHECK_GE(index, 0);
   DCHECK_LE(index, h_char_array->GetLength());
   DCHECK_LE(end - start, h_char_array->GetLength() - index);
   string->GetChars(start, end, h_char_array, index);
diff --git a/runtime/jni_internal_test.cc b/runtime/jni_internal_test.cc
index e9909352ec..a421c340ae 100644
--- a/runtime/jni_internal_test.cc
+++ b/runtime/jni_internal_test.cc
@@ -679,12 +679,8 @@ TEST_F(JniInternalTest, AllocObject) {
   ASSERT_TRUE(env_->IsInstanceOf(o, c));
   // ...whose fields haven't been initialized because
   // we didn't call a constructor.
-  if (art::mirror::kUseStringCompression) {
-    // Zero-length string is compressed, so the length internally will be -(1 << 31).
-    ASSERT_EQ(-2147483648, env_->GetIntField(o, env_->GetFieldID(c, "count", "I")));
-  } else {
-    ASSERT_EQ(0, env_->GetIntField(o, env_->GetFieldID(c, "count", "I")));
-  }
+  // Even with string compression empty string has `count == 0`.
+  ASSERT_EQ(0, env_->GetIntField(o, env_->GetFieldID(c, "count", "I")));
 }
 
 TEST_F(JniInternalTest, GetVersion) {
@@ -895,11 +891,12 @@ TEST_F(JniInternalTest, FromReflectedField_ToReflectedField) {
   // Make sure we can actually use it.
   jstring s = env_->NewStringUTF("poop");
   if (mirror::kUseStringCompression) {
-    // Negative because s is compressed (first bit is 1)
-    ASSERT_EQ(-2147483644, env_->GetIntField(s, fid2));
+    ASSERT_EQ(mirror::String::GetFlaggedCount(4, /* compressible */ true),
+              env_->GetIntField(s, fid2));
     // Create incompressible string
     jstring s_16 = env_->NewStringUTF("\u0444\u0444");
-    ASSERT_EQ(2, env_->GetIntField(s_16, fid2));
+    ASSERT_EQ(mirror::String::GetFlaggedCount(2, /* compressible */ false),
+              env_->GetIntField(s_16, fid2));
   } else {
     ASSERT_EQ(4, env_->GetIntField(s, fid2));
   }
diff --git a/runtime/mirror/string-inl.h b/runtime/mirror/string-inl.h
index d94b39fba7..6870fda075 100644
--- a/runtime/mirror/string-inl.h
+++ b/runtime/mirror/string-inl.h
@@ -106,9 +106,7 @@ class SetStringCountAndValueVisitorFromCharArray {
     string->SetCount(count_);
     const uint16_t* const src = src_array_->GetData() + offset_;
     const int32_t length = String::GetLengthFromCount(count_);
-    bool compressible = kUseStringCompression && String::GetCompressionFlagFromCount(count_);
-    DCHECK(!compressible || kUseStringCompression);
-    if (compressible) {
+    if (kUseStringCompression && String::IsCompressed(count_)) {
       for (int i = 0; i < length; ++i) {
         string->GetValueCompressed()[i] = static_cast<uint8_t>(src[i]);
       }
@@ -126,7 +124,8 @@ class SetStringCountAndValueVisitorFromCharArray {
 // Sets string count and value in the allocation code path to ensure it is guarded by a CAS.
 class SetStringCountAndValueVisitorFromString {
  public:
-  SetStringCountAndValueVisitorFromString(int32_t count, Handle<String> src_string,
+  SetStringCountAndValueVisitorFromString(int32_t count,
+                                          Handle<String> src_string,
                                           int32_t offset) :
     count_(count), src_string_(src_string), offset_(offset) {
   }
@@ -137,8 +136,7 @@ class SetStringCountAndValueVisitorFromString {
     ObjPtr<String> string = ObjPtr<String>::DownCast(obj);
     string->SetCount(count_);
     const int32_t length = String::GetLengthFromCount(count_);
-    bool compressible = kUseStringCompression && String::GetCompressionFlagFromCount(count_);
-    DCHECK(!compressible || kUseStringCompression);
+    bool compressible = kUseStringCompression && String::IsCompressed(count_);
     if (src_string_->IsCompressed()) {
       const uint8_t* const src = src_string_->GetValueCompressed() + offset_;
       memcpy(string->GetValueCompressed(), src, length * sizeof(uint8_t));
@@ -209,8 +207,7 @@ inline String* String::Alloc(Thread* self, int32_t utf16_length_with_flag,
                              gc::AllocatorType allocator_type,
                              const PreFenceVisitor& pre_fence_visitor) {
   constexpr size_t header_size = sizeof(String);
-  const bool compressible = kUseStringCompression &&
-                            String::GetCompressionFlagFromCount(utf16_length_with_flag);
+  const bool compressible = kUseStringCompression && String::IsCompressed(utf16_length_with_flag);
   const size_t block_size = (compressible) ? sizeof(uint8_t) : sizeof(uint16_t);
   size_t length = String::GetLengthFromCount(utf16_length_with_flag);
   static_assert(sizeof(length) <= sizeof(size_t),
@@ -245,7 +242,7 @@ inline String* String::Alloc(Thread* self, int32_t utf16_length_with_flag,
 
 template <bool kIsInstrumented>
 inline String* String::AllocEmptyString(Thread* self, gc::AllocatorType allocator_type) {
-  const int32_t length_with_flag = String::GetFlaggedCount(0);
+  const int32_t length_with_flag = String::GetFlaggedCount(0, /* compressible */ true);
   SetStringCountVisitor visitor(length_with_flag);
   return Alloc<kIsInstrumented>(self, length_with_flag, allocator_type, visitor);
 }
@@ -255,10 +252,9 @@ inline String* String::AllocFromByteArray(Thread* self, int32_t byte_length,
                                           Handle<ByteArray> array, int32_t offset,
                                           int32_t high_byte, gc::AllocatorType allocator_type) {
   const uint8_t* const src = reinterpret_cast<uint8_t*>(array->GetData()) + offset;
-  const bool compressible = kUseStringCompression && String::AllASCII<uint8_t>(src, byte_length)
-                                            && (high_byte == 0);
-  const int32_t length_with_flag = (compressible) ? String::GetFlaggedCount(byte_length)
-                                                  : byte_length;
+  const bool compressible =
+      kUseStringCompression && String::AllASCII<uint8_t>(src, byte_length) && (high_byte == 0);
+  const int32_t length_with_flag = String::GetFlaggedCount(byte_length, compressible);
   SetStringCountAndBytesVisitor visitor(length_with_flag, array, offset, high_byte << 8);
   String* string = Alloc<kIsInstrumented>(self, length_with_flag, allocator_type, visitor);
   return string;
@@ -272,7 +268,7 @@ inline String* String::AllocFromCharArray(Thread* self, int32_t count,
   DCHECK_GE(array->GetLength(), count);
   const bool compressible = kUseStringCompression &&
                             String::AllASCII<uint16_t>(array->GetData() + offset, count);
-  const int32_t length_with_flag = (compressible) ? String::GetFlaggedCount(count) : count;
+  const int32_t length_with_flag = String::GetFlaggedCount(count, compressible);
   SetStringCountAndValueVisitorFromCharArray visitor(length_with_flag, array, offset);
   String* new_string = Alloc<kIsInstrumented>(self, length_with_flag, allocator_type, visitor);
   return new_string;
@@ -284,8 +280,7 @@ inline String* String::AllocFromString(Thread* self, int32_t string_length, Hand
   const bool compressible = kUseStringCompression &&
       ((string->IsCompressed()) ? true : String::AllASCII<uint16_t>(string->GetValue() + offset,
                                                                     string_length));
-  const int32_t length_with_flag = (compressible) ? String::GetFlaggedCount(string_length)
-                                                  : string_length;
+  const int32_t length_with_flag = String::GetFlaggedCount(string_length, compressible);
   SetStringCountAndValueVisitorFromString visitor(length_with_flag, string, offset);
   String* new_string = Alloc<kIsInstrumented>(self, length_with_flag, allocator_type, visitor);
   return new_string;
@@ -311,7 +306,7 @@ inline int32_t String::GetHashCode() {
 template<typename MemoryType>
 bool String::AllASCII(const MemoryType* const chars, const int length) {
   for (int i = 0; i < length; ++i) {
-    if (chars[i] > 0x80) {
+    if (chars[i] >= 0x80) {
       return false;
     }
   }
diff --git a/runtime/mirror/string.cc b/runtime/mirror/string.cc
index 4336aa1ee3..0ab0bd6794 100644
--- a/runtime/mirror/string.cc
+++ b/runtime/mirror/string.cc
@@ -95,8 +95,7 @@ String* String::AllocFromStrings(Thread* self, Handle<String> string, Handle<Str
   gc::AllocatorType allocator_type = Runtime::Current()->GetHeap()->GetCurrentAllocator();
   const bool compressible = kUseStringCompression &&
       (string->IsCompressed() && string2->IsCompressed());
-  const int32_t length_with_flag = compressible ? String::GetFlaggedCount(length + length2)
-                                                : (length + length2);
+  const int32_t length_with_flag = String::GetFlaggedCount(length + length2, compressible);
 
   SetStringCountVisitor visitor(length_with_flag);
   ObjPtr<String> new_string = Alloc<true>(self, length_with_flag, allocator_type, visitor);
@@ -132,8 +131,7 @@ String* String::AllocFromUtf16(Thread* self, int32_t utf16_length, const uint16_
   gc::AllocatorType allocator_type = Runtime::Current()->GetHeap()->GetCurrentAllocator();
   const bool compressible = kUseStringCompression &&
                             String::AllASCII<uint16_t>(utf16_data_in, utf16_length);
-  int32_t length_with_flag = (compressible) ? String::GetFlaggedCount(utf16_length)
-                                            : utf16_length;
+  int32_t length_with_flag = String::GetFlaggedCount(utf16_length, compressible);
   SetStringCountVisitor visitor(length_with_flag);
   ObjPtr<String> string = Alloc<true>(self, length_with_flag, allocator_type, visitor);
   if (UNLIKELY(string == nullptr)) {
@@ -169,8 +167,7 @@ String* String::AllocFromModifiedUtf8(Thread* self,
                                       int32_t utf8_length) {
   gc::AllocatorType allocator_type = Runtime::Current()->GetHeap()->GetCurrentAllocator();
   const bool compressible = kUseStringCompression && (utf16_length == utf8_length);
-  const int32_t utf16_length_with_flag = (compressible) ? String::GetFlaggedCount(utf16_length)
-                                                        : utf16_length;
+  const int32_t utf16_length_with_flag = String::GetFlaggedCount(utf16_length, compressible);
   SetStringCountVisitor visitor(utf16_length_with_flag);
   ObjPtr<String> string = Alloc<true>(self, utf16_length_with_flag, allocator_type, visitor);
   if (UNLIKELY(string == nullptr)) {
diff --git a/runtime/mirror/string.h b/runtime/mirror/string.h
index 6ce75bccb6..95b6c3e76b 100644
--- a/runtime/mirror/string.h
+++ b/runtime/mirror/string.h
@@ -33,6 +33,10 @@ namespace mirror {
 
 // String Compression
 static constexpr bool kUseStringCompression = false;
+enum class StringCompressionFlag : uint32_t {
+    kCompressed = 0u,
+    kUncompressed = 1u
+};
 
 // C++ mirror of java.lang.String
 class MANAGED String FINAL : public Object {
@@ -78,7 +82,6 @@ class MANAGED String FINAL : public Object {
   void SetCount(int32_t new_count) REQUIRES_SHARED(Locks::mutator_lock_) {
     // Count is invariant so use non-transactional mode. Also disable check as we may run inside
     // a transaction.
-    DCHECK_LE(0, (new_count & INT32_MAX));
     SetField32<false, false>(OFFSET_OF_OBJECT_MEMBER(String, count_), new_count);
   }
 
@@ -175,7 +178,7 @@ class MANAGED String FINAL : public Object {
 
   template<VerifyObjectFlags kVerifyFlags = kDefaultVerifyFlags>
   bool IsCompressed() REQUIRES_SHARED(Locks::mutator_lock_) {
-    return kUseStringCompression && GetCompressionFlagFromCount(GetCount());
+    return kUseStringCompression && IsCompressed(GetCount());
   }
 
   bool IsValueNull() REQUIRES_SHARED(Locks::mutator_lock_);
@@ -183,16 +186,27 @@ class MANAGED String FINAL : public Object {
   template<typename MemoryType>
   static bool AllASCII(const MemoryType* const chars, const int length);
 
-  ALWAYS_INLINE static bool GetCompressionFlagFromCount(const int32_t count) {
-    return kUseStringCompression && ((count & (1u << 31)) != 0);
+  ALWAYS_INLINE static bool IsCompressed(int32_t count) {
+    return GetCompressionFlagFromCount(count) == StringCompressionFlag::kCompressed;
+  }
+
+  ALWAYS_INLINE static StringCompressionFlag GetCompressionFlagFromCount(int32_t count) {
+    return kUseStringCompression
+        ? static_cast<StringCompressionFlag>(static_cast<uint32_t>(count) & 1u)
+        : StringCompressionFlag::kUncompressed;
   }
 
-  ALWAYS_INLINE static int32_t GetLengthFromCount(const int32_t count) {
-    return kUseStringCompression ? (count & INT32_MAX) : count;
+  ALWAYS_INLINE static int32_t GetLengthFromCount(int32_t count) {
+    return kUseStringCompression ? static_cast<int32_t>(static_cast<uint32_t>(count) >> 1) : count;
   }
 
-  ALWAYS_INLINE static int32_t GetFlaggedCount(const int32_t count) {
-    return kUseStringCompression ? (count | (1u << 31)) : count;
+  ALWAYS_INLINE static int32_t GetFlaggedCount(int32_t length, bool compressible) {
+    return kUseStringCompression
+        ? static_cast<int32_t>((static_cast<uint32_t>(length) << 1) |
+                               (static_cast<uint32_t>(compressible
+                                                          ? StringCompressionFlag::kCompressed
+                                                          : StringCompressionFlag::kUncompressed)))
+        : length;
   }
 
   static Class* GetJavaLangString() REQUIRES_SHARED(Locks::mutator_lock_) {
diff --git a/test/021-string2/src/Main.java b/test/021-string2/src/Main.java
index a848fba6f5..51351e1835 100644
--- a/test/021-string2/src/Main.java
+++ b/test/021-string2/src/Main.java
@@ -431,6 +431,22 @@ public class Main {
                 "\u0440\u0440\u0440\u0440\u0440\u0440z\u0440",
                 "\u0440\u0440\u0440\u0440\u0440\u0440\u0440z\u0440",
                 "\u0440\u0440\u0440\u0440\u0440\u0440\u0440\u0440z\u0440",
+                "\u0000",
+                "\u0000\u0000",
+                "\u0000\u0000\u0000",
+                "\u0000\u0000\u0000\u0000",
+                "\u0000\u0000\u0000\u0000\u0000",
+                "\u0000\u0000\u0000\u0000\u0000\u0000",
+                "\u0000\u0000\u0000\u0000\u0000\u0000\u0000",
+                "\u0000\u0000\u0000\u0000\u0000\u0000\u0000\u0000",
+                "\u0000z\u0000",
+                "\u0000\u0000z\u0000",
+                "\u0000\u0000\u0000z\u0000",
+                "\u0000\u0000\u0000\u0000z\u0000",
+                "\u0000\u0000\u0000\u0000\u0000z\u0000",
+                "\u0000\u0000\u0000\u0000\u0000\u0000z\u0000",
+                "\u0000\u0000\u0000\u0000\u0000\u0000\u0000z\u0000",
+                "\u0000\u0000\u0000\u0000\u0000\u0000\u0000\u0000z\u0000",
         };
         String[] suffixes = {
                 "",
@@ -458,30 +474,40 @@ public class Main {
                     String full = p + c + s;
                     int expX = (c.isEmpty() || c.charAt(0) != 'x') ? -1 : p.length();
                     int exp0440 = (c.isEmpty() || c.charAt(0) != '\u0440') ? -1 : p.length();
+                    int exp0000 = (c.isEmpty() || c.charAt(0) != '\u0000') ? -1 : p.length();
                     Assert.assertEquals(expX, $noinline$indexOf(full, 'x'));
                     Assert.assertEquals(exp0440, $noinline$indexOf(full, '\u0440'));
+                    Assert.assertEquals(exp0000, $noinline$indexOf(full, '\u0000'));
                     Assert.assertEquals(expX, $noinline$indexOf(full, 'x', -1));
                     Assert.assertEquals(exp0440, $noinline$indexOf(full, '\u0440', -1));
+                    Assert.assertEquals(exp0000, $noinline$indexOf(full, '\u0000', -1));
                     Assert.assertEquals(-1, $noinline$indexOf(full, 'x', full.length() + 1));
                     Assert.assertEquals(-1, $noinline$indexOf(full, '\u0440', full.length() + 1));
+                    Assert.assertEquals(-1, $noinline$indexOf(full, '\u0000', full.length() + 1));
                     for (int from = 0; from != full.length(); ++from) {
                         final int eX;
                         final int e0440;
+                        final int e0000;
                         if (from <= p.length()) {
                             eX = expX;
                             e0440 = exp0440;
+                            e0000 = exp0000;
                         } else if (from >= p.length() + c.length()) {
                             eX = -1;
                             e0440 = -1;
+                            e0000 = -1;
                         } else if (full.charAt(from) == 'z') {
                             eX = (full.charAt(from + 1) != 'x') ? -1 : from + 1;
                             e0440 = (full.charAt(from + 1) != '\u0440') ? -1 : from + 1;
+                            e0000 = (full.charAt(from + 1) != '\u0000') ? -1 : from + 1;
                         } else {
                             eX = (full.charAt(from) != 'x') ? -1 : from;
                             e0440 = (full.charAt(from) != '\u0440') ? -1 : from;
+                            e0000 = (full.charAt(from) != '\u0000') ? -1 : from;
                         }
                         Assert.assertEquals(eX, $noinline$indexOf(full, 'x', from));
                         Assert.assertEquals(e0440, $noinline$indexOf(full, '\u0440', from));
+                        Assert.assertEquals(e0000, $noinline$indexOf(full, '\u0000', from));
                     }
                 }
             }